This question could have been asked in the Tech Talk thread, but I'll answer it here.
True HD Video begins with a resolution of 1280x720 pixels and is in the 16:9 aspect ratio. "Full HD" has a resolution of 1920x1080 pixels, the maximum resolution covered by HDTV.
Even higher resolutions are used for digital cinema projectors, for example.
Anamorphic compression to 720x480 pixels is used to downgrade the content
to NTSC displays, or by websites to reduce file sizes. This resolution is considered SD (Standard Definition).
For a detailed description I reccomend
this wikipedia article.