RealVideo is a proprietary video format developed by RealNetworks?. It was released in 1997 to accompany RealAudio. The two are usually together in a RealMedia container, with the file extension (.rm).
RealMedia is primarily a streaming media format, it can be viewed while it is being dowloaded. It is supported on a number of platforms, including Windows, Mac, Linux, Solaris, and even some mobile phones.