E6 is, briefly, the idea of capping level advancement at 6th level... you play normal 3.5 until then... and after that, giving PCs 1 feat per 5000xp earned. It includes a few special feats to make up for the fact that you're not advancing past 6th level (like ones to allow a sorcerer to expand his spell knowledge), and some other options, but it that's the core of the idea.