friends who play together. 

It used to be a derogatory term for teams who would stomp other teams who weren't as coordinated. Now it's usually applied to any group of people with a familiarity more than 12%. 

If you enjoy reading the forums and want some more info about premades and pugs and a discussion on them, check out these older posts where some of the pros and cons of Pre and Pug were discussed at length.

http://forums.impulsedriven.com/365479

http://forums.impulsedriven.com/366607

At this point, there aren't a lot premades around anymore.  You could consider the clan OMG a premade, but its really just several people on voice chat with varied skill levels - so, its generally not like what it used to be where you'd have the top players on 1 team vs another team.