RVMB is a file type developed by Real Networks and stands for RealMedia Variable Bit Rate file. RMVB is quite similar to MPEG4 files and differs from the RealMedia format which encodes at a constant value. XVID and DIVX are other types of variable bitrate files. RMVB files are quite common in Asia but less prevalent in Europe and the Americas.
Variable bit rate files alter the amount of data for a given amount of time. This allows the bitrate to be increased for more complex elements of the file, and decreased for less complex parts. This means the storage requirements of the file are less with a negligible decrease in image and audio quality. The disadvantage of this type of file is that the encoding process is longer compared to a constant bitrate file.
MP3 is a digital music format which allows CD tracks to be reduced to around a tenth of their normal size without a significant loss of quality. MP3 gets rid of a lot of the information recorded in a song that our ears are not able to hear and then uses complex algorithms to reduce the file size. This then enables you to get hundreds of songs on to a CD and it also has opened up a new market over the internet - the download market as download times have been significantly reduced.
The MP3 format is a lossy format. That means that an MP3 file does not contain 100% of the original audio information. Instead, MP3 files use perceptual coding. In other words, that means it removes the information that your ear doesn't notice thereby making the file smaller. The reason lossy formats are used over RAW is that RAW audio files are too large to travel over the internet at any great speed. By using lossy formats it enables even dial up users to download mp3 files at a reasonable speed. RAW file formats generally require 176,000 bytes per second compared to a lossy format which requires 17,600. The difference is massive and so are the download times.