Explain Block's

[Gargoyle]

So apart from the Card Search Engine(which seems to have its own problems) I can't find any readily available explanation of Block format, or any list of which sets it covers and when this will change. Perhaps this information is somewhere if you know where to look, but there should be a Stickied Thread in Block discussion and possibly the Cards/Rules one too.

I understand the concept of Blocks, as they exist in other CCGs, but I still think some clarification is required.