H.264 is a video compression standard that’s been around since 2003; many have championed the idea of having that codec as the specific standard for all web video. Well, that won’t be happening in Google Chrome as they’re championing the open format WebM these days, and as such dropping H.264 support from Chrome altogether.
Personally I agree with this maneuver by Google for one very simple reason: H.264 is and always has been horrible in a web browser. Outside of the browser it’s fine, but when inside, the computing power it takes to do it ("shell out" to app/plugin, start playback, potentially crash browser every time it happens) is ridiculous even compared to Flash.
WebM, if you haven’t seen it yet, is very good. Not only does WebM look great, but it also works better on older computers (a definite plus). It has support in Firefox 4, Opera 10.60 and Google Chrome 6 and above. VLC and WinAMP also support WebM.