MPEG-4 (.mp4). Moving Picture Experts Group defined in RFC 4337.

The standard video format for use in the Digital Library is MPEG-4 (.mp4). See
Video Format Best Practices

MPEG-4 is an extensive collection of compression schemas for audio and visual digital data. MPEG-4 defines compression for ultra-low bandwidth video applications such as video conferencing. The standard is based on the QuickTime file format due to its robust and extensible architecture.

H.264 (MPEG-4 Part 10) is a popular compression algorithm using this specification. It is an ultra-efficient technology that gives excellent results across a broad range of bandwidths, from 3G for mobile devices to iChat AV for video conferencing to HD for broadcast and DVD. H.264 delivers the same quality as MPEG-2 at a third to half the data rate and up to four times the frame size of MPEG-4 Part 2 at the same data rate.

MP4 video format can be played using HTML5 by the Chrome, Firefox, Internet Exploer, and Safari web browsers, but not by Opera (see http://www.w3schools.com/html/html5_video.asp.

Systems such as iTunes only accept certain formats. If the video can be imported into iTunes, right-click on it, or use the top-of-screen menu to select Advanced > Create iPod or iPhone version. This uses QuickTime to convert the video in into an H.264 format playable by the iPod, at a maximum resolution of 640x480.

Video-capable iPod and iPhone devices only play video in MPEG-4 (.mp4) and H.264 codecs.

• * H.264 video, up to 1.5 Mbps, 640 x 480, 30 frames per sec., Baseline Low-Complexity Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats
• * H.264 video, up to 768 kbps, 320 x 240, 30 frames per sec., Baseline Profile up to Level 1.3 with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats
• * MPEG-4 video, up to 2.5 Mbps, 640 x 480, 30 frames per sec., Simple Profile with AAC-LC audio up to 160 kbps, 48 Khz, stereo audio in .m4v, .mp4, and .mov file formats

MP4 videos can be downloaded from YouTube in the following formats:

• * HQ18 : high-quality; ipod format; H.264 codec; 480x270 resolution; 4:3 ratio; 610 bitrate; AAC audio; add &fmt=18 to the url
• * HQ22 : [high-definition; H.264 codec; 1280x720 resolution; 19:9 ratio; AAC audio; add &fmt=22 to the url, if it is available

For more details see the Wikipedia article: MPEG-4 Part 14