It's not actually online.
There are resources for looking at the material that's in the game books online.
There's character-builder software.
There's supposed to be a fancy combination of Skype, a die roller, and a virtual battlemap that will make it easy to play online, but I'm not sure how much progress they've made on that -- WotC has a very poor track record as far as RPG-facilitating software. 3rd Edition was supposed to have fancy DM software but that turned into a much-delayed character builder instead.
Also, WotC just pulled the plug on the PDF market entirely, so you could say that they're actually less "online" than some other game companies in some respects.
Of course, you can play RPGs online using various bits of free software (though D&D can be map-intensive and a lot of the virtual battlemaps are pretty clunky right now).
-- Alex