Basically a skin is an online poker room that is licensed to provide you with access to a common network under their "branding" (or 'skin'!), For example, oPoker is a skin of the Boss Media Network. So effectively, you are playing on Boss Media, but the branding on your poker client will be oPoker.
Therefore, for the player there is not really much of a difference between each skin. The interface may look different and there might be individual promotions. When it comes to the core poker operation the differences are few. At the skin site you will find the exact same players, the same games and tables.