Originally Posted: http://minecraft.jermsmit.com/minecraft-skins-pc/

A skin refers to the texture that is placed onto a player model or mob.

The skin is divided into areas that act as the surface area of the character (For example, there’s the front head area, left leg area, etc.). A skin will only allow solid color; transparency is not allowed on the skin file except on the second layer, which is transparent by default; playing offline, pixels can be left free resulting in “holes” in the skin. The second layer can be used to give the character glasses, hats, or other accessories (even a bigger head).

A player can only change their character’s skin if they have purchased Minecraft. This is done on the Profile page by uploading a valid .png image file, which will then replace the default skin.

Skins also have the option of having 3 or 4 pixel wide arms, which can be changed

on the profile page as well

ref: http://minecraft.gamepedia.com/Skin

But where do I get them?

This is a good question.  You can make your own; awesome idea for those who are creative and wan’t to have something of their own.

Or you can simple download them. There are many sites online, but one must beware as some sites are there just to scam you and infect your computer with malware and worse acquire your account info.

Here are known sites I trust and would give my recommendation on using:

  1. Minecraft Skins – The Skindex – http://www.minecraftskins.com/
  2. Planet Minecraft – http://www.planetminecraft.com/resources/skins/