AVI is short for Audio Visual Interleaved. It's a computer graphics animation format used in Microsoft's video for Windows Standard, or a chunk-based container file format for storing sound and moving pictures defined by Microsoft. This format interleaves digitized video frames (or computer-generated frames) and synchronized audio in one file.

AVI is a common video file format with the extension name of .avi. The video quality can be very good at smaller resolutions, but files tend to be rather large. An AVI file can use different codecs and formats, so there is no set format for an AVI file unlike for example standard VCD video which sets a standard for resolution, bit rates, and codecs used.
In saying of AVI is a chunk-based container file format, we are saying that AVI is a derivative of the Resource Interchange File Format (RIFF), which divides a file's data into blocks, or "chunks." Each "chunk" is identified by a FourCC tag. An AVI file takes the form of a single chunk in a RIFF formatted file, which is then subdivided into two mandatory "chunks" and one optional "chunk". [P.S. FourCC (literally, four-character code) is a sequence of four bytes used to uniquely identify data formats. Later, this idea is used to identify compressed data types in QuickTime and DirectShow.

The first sub-chunk is identified by the "hdrl" tag. This sub-chunk is the file header and contains metadata about the video, such as its width, height and frame rate. The second sub-chunk is identified by the "movi" tag. This chunk contains the actual audio/visual data that make up the AVI movie. The third optional sub-chunk is identified by the "idx1" tag which indexes the offsets of the data chunks within the file.

By way of the RIFF format, the audio/visual data contained in the "movi" chunk can be encoded or decoded by software called a codec. Upon creation of the file, the codec translates between raw data and the (compressed) data format used inside the chunk. An AVI file may carry audio/visual data inside the chunks in virtually any compression scheme, including Full Frame (Uncompressed), Intel Real Time (Indeo), Cinepak, Motion JPEG, Editable MPEG, VDOWave, ClearVideo / RealVideo, QPEG, and MPEG-4 Video.

FLV is short for Flash video. Flash video is the name of a file format used to deliver video over the Internet using Adobe Flash Player (formerly known as Macromedia Flash Player) version 6, 7, 8, or 9. Until version 9 update 2 of the Flash Player, Flash video referred to a proprietary file format, having the extension .flv. The most recent public release of Flash Player supports H.264 video. Flash video content can also be embedded within SWF files. Notable users of the Flash video format include YouTube, Google Video, Reuters.com, Yahoo! Video, and many television news operations are also using Flash video on their websites.

Flash video is viewable on most operating systems, via the wide availability of Adobe Flash Player and web browser plugin, or one of several third-party programs such as MPlayer, VLC media player, Quicktime, or any player which uses DirectShow filters (such as Media Player Classic, Windows Media Player, and Windows Media Center).

Generally, in order to help well interpret what is FLV, we can lay out the primary features of it right here:

1. It's the best format for the compressed video in Flash (best quality and smallest file size).

2. It's native format to Adobe Flash (FLV is not used for any other software).

3. It can be created from AVI, MOV, WMV, MP3, MP4, 3GP, DVD (using our converter tools like Flash Video MX, MX Pro, and MX SDK and software programs from other companies).

4. Generally, almost all the videos on the web with a Flash Player are in FLV format. (Videos on YouTube, Google Video, Yahoo! Video are all of this kind.)