All of the export format choices are H.264 (and so any of them are suitable for embedding in a web page for Safari), each with slightly different sizes and framerates. Roughly they mean:
iPod: 640x480, 30 fps, 1.5 Mbps
iPhone: 480x360, 30 fps, 900 kbps
Apple TV: 720p up to 24 fps, otherwise 640x480, 5 Mbps
H.264: tries to match the source file bitrate and resolution, including HD
(Or if the source video is smaller, such as an old camera that can only shoot 320x240, then those numbers are lower. In other words it never scales up or changes the framerate.)
The first three are actually presets from Apple, and this technote has more details:
http://developer.apple.com/mac/library/technotes/tn2007/tn2188.html