MP4 is a container format. What this means is that, like the AVI format, MP4 files can include a video track encoded in various formats (codecs). It can also include multiple audio tracks (also supporting various codecs) and also subtitle and even chapter information. As such, there is no single method for playing back all MP4 files, since the playback is totally dependent on which codec the file is encoded with.
However, the MP4 container is most commonly used with MPEG-4 SP/ASP (available in variety of forms, including DivX, XviD) and AVC (H.264) video, with MP3, AC3 or AAC audio. AVI is still the most popular container format for forms of MPEG-4 ASP (with MP3 audio), and so most recent MP4 files you download will be H.264 with AAC audio thrown in for good measure (AAC is more compressible than MP3, in the same way AVC/H.264 is more compressible than ASP). In fact, MP4 with H.264 is so common these days, many people simply refer to it as H.264 (or x264, but that's slight misnomer as it refers to a very popular H.264 encoder).
To playback MP4 files under Windows, you'll need several things that makes up a typical Directshow based playback method. You'll first thing you need is a MP4 splitter/parser to separate the MP4 file into its separate audio, video (chapters, subtitles) elements.
The next thing you need a decoder (or decoders) to support whichever codec the video/audio are encoded in. This can usually be easily achieved using a universal decoder like ffdshow (universal because it supports practically every format there is, including DivX/XviD/H.264).
And of course, you'll need a player software which supports the aforementioned splitters and decoders. Again, this is easily available in the form of Media Player Classic, an open source multimedia player.
For those not using Windows, there are alternative software (some also work under Windows) like Apple QuickTime, MPlayer, VideoLAN, xine...
Both the Xbox 360 and PS3 also support MP4/H.264 playback. Guides are available here to show you how to make and play MP4/H.264 files on the Xbox 360 and PS3. In addition to consoles, many other devices such as the Apple TV both support MP4/H.264, and there are one-click encoders specifically available to make compatible files for these devices (see the iPhone and Apple TV links for guides).
There are other kinds of MP4 files, MP4 files converted from DVD or other kinds of videos. To ensure that you can get MP4 that can be played appropriately, you must choose DVD to MP4 converter or DVD to iPod Converter carefully.

2009 - 02 -20