Real Media, real audio and RMVB are the popular formats for streaming content on the Internet
RM
RM stands for Real Media. Real Media is one of the most popular formats for streaming content on the Internet. RealMedia includes the RealAudio codec for sound clips and RealVideo codec for movies. RealAudio and RealVideo files are often given the common RealMedia ".rm" file extension. RealMedia files are often heavily compressed so they can stream over dial-up Internet connections. It mainly includes RealAudio, RealVideo and RealFlash.
RA
RealAudio is a proprietary audio format developed by RealNetworks. It uses a variety of audio codecs, ranging from low-bitrate formats that can be used over dialup modems, to high-fidelity formats for music. It can also be used as a streaming audio format, which is played at the same time as it is downloaded.
RMVB
RealMedia Variable BitRate (RMVB) is a variable bitrate extension of the RealMedia multimedia container format developed by RealNetworks. Compared with former generation RM format, the picture of RMVB is much clear.