I have only a mild understanding of what is going on with the X-Men these days. There are apparently only 157 or so mutants left in the world after the Scarlet Witch de-powered most of them. Here's the plot synopsis for the anniversary issue:
Sentinels? What? And Magneto? And is that the Master Mold? What the hell is going on in San Francisco now that the Uncanny X-Men have relocated there? They’ve got a new Headquarters and a new status quo as the gauntlet is thrown for a new era of mutantkind. It’s all here in the double-sized anniversary issue that sets up the plotlines that will be developed over the next year in UNCANNY. And it’s all brought to you by the hot writing team of Ed Brubaker & Matt Fraction (Immortal Iron Fist) and fan favorite artists Greg Land (ULTIMATE FANTASTIC FOUR, X-MEN: FREE COMIC BOOK DAY) and Terry Dodson (SPIDER-MAN/BLACK CAT, Wonder Woman). New creators, new home. The future begins now.
You can look at the first few pages of Uncanny X-Men #500 over at Comic Book Resources.