[Ducky]

I'm not an expert on them. but from what I've seen there are 'blocks' which consist of 3 different sets. the whole block will have an overall theme to it, and a story in the Magic Universe. I think there is also a New game mechanic added for each block, which is features in the 3 sets within it.

Take the Innistrad block for example. The first set being Innistrad, then Dark Asention, and now Avacyn Restored. The first two sets concentrated a lot on dark evil stuff. Mainly werewolves, vampires and zombies. the 3rd, up coming, set is more about angels and the banishing of the darkness from the previous two sets in the block.
The Game mechanic that was introduced in the Innistrad block is the double sided flipping cards.