Imagine you live in the New York City of the standard, or 616, Marvel Universe. Almost every day, heroes and villains fight each other, and one of the top heroes in town is Peter Parker, the Amazing Spider-Man. But civilians like you and I usually don't get involved in the action (apart from being threatened by the villains and/or saved by the heroes). Now imagine that almost everyone in New York City is getting spider powers like Peter. Most people use these powers for their own fun and profit, while others learn that with great power... (you know the rest). And this is only stage 1 of the infestation.
An action- and fun-packed comic event you won't want to miss. For me, it was easily the Marvel event, no, even the comics event of the year 2011 (better than Fear Itself or even the mini-event X-Men Schism). Make sure to grab some of the tie-ins as well when they become available as trade paperbacks. I particularly enjoyed Cloak & Dagger, Deadly Hands of Kung Fu, and The Amazing Spider-Girl.