What is a Skin?
Skins allow you to change the appearance of your in-game character. This change will be visible to players on any multiplayer server you connect to! If you'd like, you can design your own skin, but if you're artistically challenged, there are plenty of great sites to download skins listed below.
Installing a Skin
Once your skin is designed or downloaded, go to Minecraft's official site and login. You will see a Profile link appear in the header next to Help. Click that. On the Profile page, there is a section entitled "Change Your Skin".