A premade is a Battlegrounds group that joined the battlegrounds as a group. This is for the convenience of people who want to play together in the same battlegrounds instance instead of being forked into different ones.
Some realms refer to this simply as a Raid or BG Raid.
Premade groups often come with the advantage of routine, and everyone knowing where to go and what to do. Sometimes the premade groups will be from PvP guilds and even come with their own Ventrilo server for faster communication and response. Unfortunately, this heavily takes the fairness out of playing the battlegrounds for the other team since they're most likely regular random players whose communication is the chat box.
The only obvious characteristic of a premade is the players having the same server name on the roster (shown by hitting SHIFT + Space).
How to avoid a premade
If you are on a team against a premade, and your group knows it you'll most likely feel like you are playing as the sacrificial lambs. To avoid fighting against a premade PvP group, join all the possible instances where one joins as a group. (Warsong Gulch, Arathi Basin, Eye of the Storm, and also Alterac Valley as of patch 2.4, meaning all the battlegrounds). If you have a preference for the battlegrounds you want to run, then for the other two join a specific game with a random number. This is to keep all instance queues from starting their two minute countdown for you to join while you're still evaluating your current BG. Choose whichever instance you feel like joining when it appears. View the PvP roster. If all the players are from the same server, they are most likely a premade group. You can get yourself out of the instance without dealing with Deserter status by waiting for one of the other battlegrounds queues to notify you on an open game and join those instead. Be careful if you use the 3rd instance as your last lifeline because you might be lucky enough to land yourself into that premade battle in which case you play against the odds or quit and take Deserter status.