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, that is played at the same time as it is downloaded. In the past, many internet radio stations used RealAudio to stream their programming over the internet in real time.
In recent years, however, the format has become less common and has given way to more popular audio formats. It is used heavily by the BBC websites, though its use on the site is being phased out.