Money... And the internet. The writers get a certain amount of money from every time a show that they worked on gets aired. That means reruns too. The studios (NBC, FOX, etc...) get money from advertising. Thats how its been since before TV was more than 2 channels.
But, the studios have been putting episodes of shows on the internet, paid for by advertisers (you have to sit through an ad before watching the show).
The problem was, was that writers were not getting any payment for the shows that were put online. The shows were airing, making money, and the writers weren't getting a cent.
Hence the strike. They were also trying to hammer out guidelines for the future.
I might be off on some of this stuff... but thats generally what it was about.