What is RA?

RealAudio (RA) is a proprietary audio format developed by RealNetworks and first released in 1995. It was developed as a streaming media format. That means, it can be played while it is downloading. RealAudio uses a variety of audio codecs, ranging from low-bitrate formats that can be used over dialup modems, to high-fidelity formats for music.
RealAudio is using less common and popular as past, due to its own playing restrict and other multiple formats produced, now only BBC World Service still using this format.