Definition of skin
Skin means "skin" in French. In the poker world, when we talk about a skin, we are talking about a site belonging to the same network. For example, a poker network (which we will call Network) could own Game Room 1, Game Room 2 and Game Room 3 and present them with different skins. It may look like they are two different game rooms, but they are connected to the same network. Even though the name of the poker room will not be the same and the promotions will not be the same, you will probably be playing against the same players.
Different skins for legal reasons
If we take Canada as an example, there is poker offered in Quebec and British Columbia. Each province offers poker, but does not own the game software. Therefore, they have an agreement with a network (in this case, GTECH) that will offer a specific skin for Quebec (called OK Poker) and another skin for British Columbia with a game room called Play Now. Players on the OK Poker skin will play against players in British Columbia since it is the same skin.