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Skin is used to protect the body from damage (physical or sunlight), to keep it from losing water, to regulate temperature, to sense pressure and heat, to excrete some waste products, generate vitamin D, store water and fat, and generally to stop your innards from becoming outards.
It is the largest organ in the body and can weigh up to 15% of your total body weight. It has a huge number of nerves and capillaries packed into the smallest spaces, and is able to heal itself remarkably well.
But, it's not immortal. Your skin will heal itself pretty well if you cut it, but the healing sometimes leaves scars.
As you age, your skin gets thinner and starts losing the ability to heal itself.
Skin is also vulnerable to sunlight, which can cause skin cancer. Skin cancer is by far the most common cancer in the world, accounting for 40% of all cancers, but it rarely causes death.
More than 90% of all skin cancers are caused by exposure to ultraviolet light (the sun), making it very obvious that if you avoid exposing yourself to too much sunlight, then you will probably not get skin cancer.
When skin is damaged too much to heal (large wounds or burns, for example), doctors will try to fix the problem by either pulling the skin around the wound and suturing it closed, or if the wound is too large, will transplant from another area on the body to the wounded area. The donation will be from an area that can be easily pulled tight afterwards and usually the only sign afterwards on the donor area is a thin scar. This kind of transplant is called a skin graft. The first skin graft was performed in India, 2000 years ago.
Recent developments will let biotechnology companies grow sheets of skin to fit the wound, from a small biopsy sample, allowing you to graft without needing a donor. One company, PolarityTE, claims to be able to produce completely viable human skin, including hair follicles, a product which is going to human trial in the third quarter of 2017.
In 2016, a biopsy of some skin which had the disease epidermolysis bullosa was taken from a young boy. That's a disease which loosens the outer skin's connection to the inner layers, resulting in blisters, tearing of the skin, sepsis, and usually an early death through complications. The sample was then gene-corrected using a viral delivery mechanism, correcting the defective LAMB3 gene. The corrected skin was then grown to one meter squared, and grafted back onto the boy, replacing 80% of his total skin. He now exhibits no sign at all of the diseases and is capable of normal rough play just like any other child.