Title says it all. This other question indicates that the second line with an E on the debug screen is describing the entities. What is an entity?
My guess, (based upon observations) is that it is any MOB (friendly or hostile) and any items. The "number rendered" is really the number in the field of view (i.e. the solid angle determed by the "view port" into the minecraft world, while the total number is the entire number currently loaded in memory. If you're standing still, that'd be all the mobs/items in a roughly 200x200 (in X and Z) square centered on the character? (I'm assuming single player vanilla minecraft here.)