Collagen is a protein found in animal tissues. Specifically, it's the most common protein found in connective tissue, which is the stuff outside of your cells that holds them all together. Collagen is actually the protein that makes up tendons, cartilage, and even bone. It's found in blood vessels, in the gut, and even in the teeth. It's not just in humans, either; it's the "glue" that holds every animal together.
Glue isn't just an analogy here. The word collagen comes from the Greek root Kolla, which means glue.
Since collagen is such a prominent protein in the body, and since it has been used all the way back through antiquity for everything from nutrition to medication to creating glues, it is very well studied. Unlike many other supplements, the benefits of collagen have a firm basis in science.
1. Collagen Can Improve Your Skin
Collagen is one of the primary components of your skin, holding your skin cells together. As you get older, your body produces less and less collagen to replace what is lost, which is why skin dries out, wrinkles, loses elasticity, and experiences other issues. 
Taking collagen as a supplement has been proven in double-blind controlled studies to have a beneficial effect on the skin. In particular, taking an oral supplement of collagen protein peptides had a beneficial effect on skin elasticity. Numerous other studies have followed showing benefits to moisture levels and skin hydration in general, as well as other wrinkle-reducing properties.
Taking collagen supplements may also improve your skin's ability to replace other skin proteins, like elastin and fibrillin, though these have been less well studied.
These studied primarily concern themselves with oral supplements of collagen protein, rather than crèmes or salves. It's worth mentioning that topical applications can cause allergic reactions, though it's easy to test for this before applying skin-based applications for any length of time.
2. Collagen Can Help Improve Healing
Again, since collagen is such a critical protein as a structural element of the body, it has many uses in modern medicine. "Scaffolds" made of collagen protein are used for healing after major surgeries or injuries that impact the skin, such as skin grafts or large burns. Collagen is commonly used as a structural basis for healing widespread tissue damage, helping to form a base from which other forms of protein can grow and anchor themselves. In addition to reconstructive surgery, this makes it ideal for cosmetic surgeries as well.
Fun fact: the collagen most commonly used in medicines and supplements comes from bovine sources, that is, cows. Great care is taken to make sure that the animals used for this process are taken from "closed herds" or from countries where mad cow disease has never been reported, to ensure the safety of the collagen produced.
Since some people's bodies reject collagen from animal sources, some collagen is actually harvested from humans! Some human remains are donated for this purpose, and some surgical operations, rather than discard removed tissue, process it for its collagen. This is typically voluntary, of course, though some issues have risen in the past. Modern collagen is much more strictly controlled than in the past, to eliminate these issues in particular.
3. Collagen Helps Alleviate Joint Pain
Collagen is a critical component in both cartilage and tendons, both of which are crucial elements of your joints. Commonly injured joints like knees, shoulders, and hips tend to deteriorate over time as you get older. In part this is simply due to the natural wear and tear on those joints, but it's also due to the gradual reduction of your body's ability to produce and replace more collagen, to heal and regenerate cartilage in your joints. 
This reduction in collagen can also lead to conditions such as osteoarthritis, as well as general joint pain. As with skin, collagen supplements can give your body more collagen to use to repair these joints.
Collagen is frequently used by older people who suffer from joint pain, but they aren't the only ones. Athletes are also known to take collagen supplements to help prevent joint injuries and pain from their physical lifestyles, as well as assisting in healing when injuries do occur.
4. Collagen Helps Prevent Bone Loss
As your body ages, it produces less and less collagen. Since collagen is such an important protein for so many parts of your body, it stands to reason that it affects just about every part of that body. Bones are no different.
While we often tend to think of bones as rigid, hard, almost stone-like scaffolds within the body, they're actually living organs. Bones help produce blood cells, they anchor tendons and muscles, and they have a crucial role to play as part of the immune system.
Over time, your bones deteriorate. Bone loss due to aging is typically called osteoporosis, which is literally "porous bones". Weaker bones lead to fractures, pain, and an increased chance of broken bones.
Taking collagen supplements can help minimize collagen loss and stimulate the production of more collagen, which your body can use to help repair your bones and strengthen them. It's not a cure-all; bones need other nutrients as well, such as calcium, to remain strong. However, it can assist with minimizing bone loss.
5. Collagen Assists Healing from Bone Grafts
In some cases, your bones deteriorate in a way that is difficult or impossible to heal. This frequently happens in the jaws around dental infections, but can occur anywhere a bone infection can reach. A bone graft is a transplant of bone material from one location to another, to replace bone that is damaged from trauma or infection. 
Collagen is a critical element in bones, but as mentioned up above, it's also a component in glue. Collagen-based glues are often used in bone grafts as a strong adhesive that promotes healing in the affected bone, as well as making it much less likely to be rejected by the body.
6. Collagen Promotes Building Muscle Mass
Unless you live a very active and very healthy lifestyle, chances are that over time, your muscles are going to deteriorate. Sarcopenia specifically is a condition of extreme muscle loss due to aging.
Collagen supplements may have a stimulating effect on the body's production of muscle proteins such as creatine, which in turn helps promote building more muscle after exercise. Collagen alone will not promote building muscle, but when used in conjunction with muscle-building exercises, studies have shown that supplements promote more muscle growth than exercise alone.
7. Collagen Benefits the Heart
We already know that collagen is a prime component in your body's tissues, and foremost in importance among those tissues is the heart. The heart is a highly specialized muscle, and the semi-rigid structure of the heart is created by a collagen-based "cardiac skeleton." No, you don't have bones in your heart; your heart simply has structure that helps it guide blood flow properly.
As with everything in your body, as you age, it gets weaker. Your heart loses some elasticity and may perform worse than it did in the past. Collagen supplements can be of some assistance to help with heart health.
8. Collagen Benefits the Cardiovascular System
Your heart isn't the only part of your cardio system that collagen can improve. Collagen is present in large quantities in blood vessels and in arteries, which can become thinner and more fragile as you age. This is why vascularity happens, why bruises become more pronounced, and is a contributing factor to heart attacks and strokes.
Collagen supplements are not a miracle cure that will resolve heart disease issues. However, stimulating the production of collagen in the body can help improve the elasticity of arteries throughout the body. Cardiovascular health is a whole-body health benefit.
9. Collagen May Help with Alzheimer's Disease
Alzheimer's Disease is a poorly understood, degenerative brain disease that leads to dementia and eventually to death. A lot of research over the last few decades has been focused on understanding Alzheimer's, and while it's still not fully understood, breakthroughs are being made every year.
Fun fact: there are actually five distinct types of collagen. One type, known as Type IV, has been shown to help prevent the amino acid plaque that forms in the brain and is thought to be at least one factor in Alzheimer's disease. Studies are still ongoing, but it's possible that this specific type of collagen may have applications in helping to prevent or minimize Alzheimer's Disease or its progression.
10. Collagen May Help Alleviate Gut Symptoms
In studies of patients with Irritable Bowel Syndrome, tests have shown those patients tend to be low in collagen type IV, the same as mentioned in the brain above. 
This has two major implications. The first is that there is an increasing amount of evidence to suggest that gut health and an appropriate gut flora is incredibly important for total body health, including brain function. The second is that collagen supplements may be able to alleviate some symptoms of gut-based disorders such as IBS.
11. Collagen Can Help Reduce Back Pain
Another very common symptom of aging is back pain. There's a lot going on in your back; your spine is the anchor for your skeletal structure and many muscles, and it houses a pillar of your central nervous system. As your body ages and deteriorates, the bones of your spine can weaken, your nervous system can misfire, your muscles can deteriorate, and more.
Collagen can help with pretty much all of these symptoms in some way. Studies of people over the age of 50 suffering from back pain showed that long-term (six month+) usage of collage supplements helped alleviate back pain significantly.
12. Collagen Helps Nail Health
Your nails are made of keratin, a hardened protein. While this is not collagen itself, collagen has a widespread effect throughout the body of increasing elasticity and suppleness while reducing brittleness. 
As you age, your nails tend to grow more and more brittle, which is why older people often suffer from broken nails more often. Taking collagen supplements helps to make your nails less brittle and more resilient, which helps prevent such damage to them.
13. Collagen Make Hair More Lustrous
Much like nails, hair is made up of keratin. It's hard to believe that both hair and nails are made up of the same stuff, but it's true. As such, the same effect collagen has on nails can be seen in hair as well. With hair, it tends to make it more supple and more resilient, which in turn allows it to grow longer with fewer split ends.
14. Collagen Can Promote Weight Loss
Now, this one isn't about the effects of collagen on the body, but rather on the mind. Collagen protein is very filling in comparison to other proteins, which means taking a collagen supplement or eating foods rich in collagen will fill you up faster and keep you satiated longer. 
When you're not as hungry all the time, you eat less, which means it's easier to maintain a caloric deficit and lose weight through exercise and other dietary factors. To be clear, collagen is not a diet or weight loss pill, but it can prove to be of some assistance.
15. Collagen is Easy to Get
Unlike some supplements, collagen is both well-studied and well-regulated. The production of collagen is a known and regulated process, so you're unlikely to be finding collagen supplements at unknown sources. Additionally, you can alter your diet by eating foods that are high in the nutrients that stimulate collagen production in the body. This includes a mixture of beef, chicken, fish, eggs, dairy, citrus, peppers, tomatoes, and greens. A general healthy diet helps your body produce more collagen, and a collagen supplement gets right to the heart of the issue by giving your body more to work with immediately.
