So, what are multimedia file formats?
And why is format conversion necessary?

ile conversion is usually required to reduce the file size or to convert to a specific file format e.g. as required by MP3 or DVD players.

There are a great number of video file formats available: AVI (audio/video interleaved), MPEG, Real Media, WMV, ASF etc.

Each file formats may contain data, which, itself, has been encoded by various methods. One such method is the use of codecs. For example, the AVI file format can be encoded with, say, DivX, DV, Indeo Video, MPEG4, etc. codecs, whereas MPEG usually have the MPEG1 and MPEG2 formats (other types, e.g. MPEG3 exist but have not gained great popularity). The most popular combination for the AVI file format is to have its video stream encoded using the DivX codec and its audio stream using MP3 file format.

There are very many audio file formats available on the market. The following are the most popular ones:: MP3 (MPEG2 Layer 3), WAV, AC3, AAC.

All codecs can be divided into two categories: those that are "lossless" and those that are "lossy". Usually, codecs fall into the lossy category and, hence, allow much greater levels of compression - up to 10-20 times the original file size.

Often, in order to shrink a multimedia file size, there is no need to change its format. Simply changing its bitrate (applicable to both video and audio formats), the frame size (the physical size of each video frame) or the frame-rate (i.e. number of video frames played per second) may achieve the reduction target. However, if after changing these settings, the file is still too large then conversion may be required. Next...