Fingerprints apparently form because our skin is made of multiple layers. The layers grow at different rates. The middle layer (basal layer) grows faster than the top (epidermis) and bottom (dermis) layers. This makes the basal layer fold into ridges. Our fingers aren't flat, so the ridges that form are irregular.
Because the cause of the ridges is down in the basal layer, surface damage doesn't change our fingerprints. As we age, they can get more faint, though. People with some specific genetic diseases that affect skin development do not develop fingerprints.
As far as we know, fingerprints are different for every individual, even identical twins. It's not impossible for people to have the same fingerprints, it's just highly unlikely. Just like if you and I each had a box of a thousand letters, it's highly unlikely that we'd make the same words and sentences from them.
Matching fingerprints found at crime scenes became common starting in the early 1900s. Now enormous databases hold millions of prints. TV shows suggest that fingerprinting is foolproof, but it takes a lot of skill to compare prints, and all sorts of things can cause smudged or smeared prints. Even computers usually just identify a group of possible matches that then have to be checked by experts.
If you find someone's fingerprint at a crime scene, does that prove the person is guilty of the crime?
Thanks for asking,