It's possible to record a raw stream of compressed video data to disk, and iMovie does just that: it takes DV data coming across FireWire and stores it in DV stream files. Most programs, though, want to see a more structured file format, and that's where "wrappers" come in. Wrapper formats add metadatadata about datato the raw video stream, making it easier for programs to manipulate video and audio information. Wrapper formats can carry information about audio sample rates, timecode, and similar information that makes it much easier for FCP to read the file correctly and keep its media synchronized. Wrappers can (and often do) contain multiple streams or tracks of information: a video track, two or more audio tracks, and a timecode track are common.
QuickTime, AVI, Windows Media, and Real are all wrapper formats.
It's important to distinguish between codecs and wrappers. A codec defines how the data is compressed, whereas a wrapper defines how the compressed data is packaged, as the name implies. You can have DV data in a QuickTime MOV file or in a Windows AVI file: it's the same video clip, only with different wrappers.
The most common wrapper format on the Mac, and the one used by FCP, is QuickTime. QuickTime is more than just a file format, it's an entire media-manipulation framework (witness QuickTime Player Pro, a very simple application that exposes some of what QuickTime can do, or the rich options in FCP's QuickTime Export dialog). QuickTime is cross-platform, having Apple implementations on Mac and on Windows, and some open-source support on Linux. Many of the codecs supported by QuickTime are also cross-platform, and the QuickTime architecture allows third parties to easily add new codecs.
QuickTime movies normally have the extension .mov. Although this isn't required on Macs, you should use it if you're going to be sharing files with PCs.
QuickTime allows for both self-contained and reference movies. A self-contained movie contains all the media it needs for playback: every frame of video, every sample of audio. A reference movie may contain some media but references other files on disk: for example, an FCP reference movie will contain any frames that FCP has to render at the time the movie is saved, but it will reference (point to) clips that don't require rendering in their original files as well as any pre-rendered render files.
AVI / Windows Media
PCs have their own native media architecture, known in its various developments and incarnations as Video for Windows, DirectShow, DirectX, and Windows Media. The common production file format normally has the extension .avi, standing for Audio Video Interleave.
QuickTime on the Mac (and, by extension, FCP) can usually open an AVI file as long as the underlying codec is compatible. DV files, for example, are usually readable. However, there are quite a few AVI codecs that don't have QuickTime equivalents, and there are some variants of the AVI file structure that may not work well with QuickTime.
When you have problems with AVI files in FCP, ask the creator of the file to ensure that a compatible codec is being used, to write out a "Video for Windows" AVI file, and to keep file sizes under 2 Gigabytes.
FCP's QuickTime infrastructure can also export AVIs: choose File > Export > Using QuickTime Conversion, and select AVI from the Format list. Don't forget to choose a codec using the Options button.
Other Windows media file types are WMA (audio) and WMV (video). These typically contain highly compressed long-GOP media using Windows proprietary codecs, and are usually not usable in FCP.
Real Media files, often with the extensions .ra and .rm, are files using Real's proprietary wrapper and codecs. Real files are encoded for final delivery using Real's proprietary player software and are not usable in FCP.
MXF and AAF
MXF, the Media Exchange Format, is an open, non-proprietary streaming file format used in some NLEs and video recorders. MXF files most often contain MPEG-2 media but may also wrap DV, DVCPROHD, and uncompressed video streams, among others. MXF files are not directly supported in FCP but may be in the future as the MXF format gains support across the industry. Currently, FCP 5 depends on Telestream's Flip4Mac MXF software to translate between MXF and QuickTime.
The Advanced Authoring Format, AAF, is "the mother of all wrapper formats" in that its aim is to wrap all aspects of a productionmedia, scripts, edit decision lists, camera metadata, color correction notes, and so onin one big bundle. It may turn out to be a great cross-platform material exchange format, but its implementation is very complex. Currently FCP has some AAF interchange capability using import/export plug-ins from Automatic Duck, useful for moving projects between FCP and Avid, among other things.
Raw Stream Files
You may also encounter "unwrapped" media files, which may or may not be usable in FCP depending on their contents. The most common ones contain DV, MPEG-2, and uncompressed media.
DV and DIF files normally contain DV streams with both video and audio. They're usable in FCP, but the audio will require rendering.
M2V and MPG files are MPEG-2 elementary and program streams. These may or may nor be usable in FCP as source clips requiring rendering, and may or may not have usable audio embedded in them.
M2T files are MPEG-2 transport streams, as imported from HDV cameras over FireWire. These aren't usable in FCP, but may contain media accessible through demultiplexing with a third-party utility such as LumiereHD.
601 files contain uncompressed video and are normally openable in FCP.
You may also encounter image sequences, sequentially-numbered still images using PICT, TGA, TIFF, BMP, DPX, or similar files formats. These are generated by computer animation programs, film telecines, or film digital-intermediate processes, and consist of individual frames of a clip in numerical order. As long as FCP understands the basic file format, it can import the sequence.
For details on working with image sequences, see "Importing Numbered Image Sequences", Final Cut Pro Help, Volume 1, Section II, page 381.