YouTube has three main formats as well as a “mobile” format.
The original format, now labeled “standard quality”, displays videos at a resolution of 320×240 pixels using the Sorenson Spark codec with mono MP3 audio.
“High quality” videos, introduced in March 2008, are shown at up to 864×480 pixels with stereo AAC sound.
In November 2008, HD support was added with a resolution of 1280 x 720 pixels, encoded with the H.264 video codec and the audio AAC stereo codec. At the same time, the YouTube player was changed from a 4:3 aspect ratio to a widescreen 16:9.
In July 2009, YouTube announced that YouTube users can now upload 3D videos. The videos can be watched in the normal way, and glasses are worn by the viewer to achieve the 3D effect.
YouTube accepts videos uploaded in most formats, including .WMV, .AVI, .MKV, .MOV, MPEG, .MP4, DivX, .FLV, and .OGG. It also supports 3GP, allowing videos to be uploaded directly from a mobile phone.
More informations are available on Wikipedia and on YouTube.