An RM file is a multimedia file saved in the RealMedia multimedia container format developed by RealNetworks.
It may store only audio in the RealAudio format, only video in the RealVideo format, or both RealAudio and RealVideo. RM files are commonly used for streaming content over the Internet such as video and Internet radio.
The media files are typically used by RealPlayer software but are also supported by other audio players or can be converted to other, more common formats by various converters. Some RealMedia files can also be played using VLC media player, which provides partial support for RM files.
RM multimedia content is streamed in constant bitrate, which means the bitrate remains the same during streaming. This is not optimal when streaming more complex segments of media data since the rate of data transfer remains constant. RealNetworks replaced the RM format with the RealMedia Variable Bitrate (RMVB) format, which enables media to be streamed in a variable bit rate, which allows data transfer at different speeds. This is useful in situations when the data transfer needs to be increased for more complex segments of media files.
RM files that only store audio may be saved as RealAudio .RA files. Also, RM files that store audio and video, or only video, may be saved as RealVideo .RV files.
NOTE: RM files may contain a URL link to a streaming media file instead of the actual audio or video data. These RM files are played back as they are downloaded.