This strange term refers to the appearance of a program's interface. By changing the skin of a program, you can make the interface look completely different, but usually still have all the same functions. It is similar to a "Theme" you may use to customize the appearance of your computer's desktop.
Skins have become particularly popular for MP3 players. Because of the simple interface of most MP3 programs, it is easy to create different looks for the interface. Other programs, such as Netscape 6, ICQ, Yahoo Messenger, and Windows Media Player also support skins. If you're one of those people who can't stand seeing the same thing over and over again, skins are for you. Some programs that support skins even provide a skin development kit that your can use to create your own. Though this allows for an unlimited amount of interface customizing, it can make it hard to recognize or use the same program on different computers.