RM is short for RealMedia, which was first created by RealNetworks. It is a popular multimedia container format widely used to stream different content over the Internet. An RM file usually contains audio or video, or both of the audio and video. An RM file is often heavily compressed so that it is appropriate for streaming over dial-up and other Internet connections.

An RM file consists of the RealAudio codec for sound clips and RealVideo codec for movies. An RM file may contain either audio tracks or video clips, and more frequently used to stream media. It can also be downloaded and played on a standalone media player. And it can be played back while it is downloading.

Like MPEG4, RM was developed primarily for streaming video over the Internet, and it is similar to other container formats. RM is a widely supported format with a high compression. For this reason, RM is commonly used by many Internet applications, such as online radio.

RM is supported by a large number of multimedia players for different architectures or platforms, including RealPlayer and the Real Alternative codec pack and Rockbox which only certain codecs contained in RM files. VLC media player includes partial support for Real Media files, and it can play some of the RA files. In July of 2009, DAP Firmware Rockbox, which is an open source, declared to support reading RM files, although can only play some certain codecs in RM files.

There are many programs that can be used to open RM files:
Windows operating system: RealPlayer for Windows, VideoLAN VLC media player, RealNetworks RealPlayer, Web browser with the RealPlayer plug-in
Linux operating system: VideoLAN VLC media player, RealNetworks RealPlayer, Web browser with the RealPlayer plug-in
Mac operating system: RealPlayer for Mac, VideoLAN VLC media player, RealNetworks RealPlayer, Web browser with the RealPlayer plug-in