Streaming media are multimedia that are sent continuously by the web server to the client. Typically they are treated as a form of presentation or broadcast that is delivered to the customers.
Many sites use streaming to server larger video (and audio) files because streaming is somewhat less arduous on the server. Streaming also allows larger files that would take hours to download completely to be viewed relatively quickly. The user begins viewing the file while the server continues sending the rest of the file in the background as they watch or listen.
Other sites use streaming media because the files are not downloaded by the customer. They are viewed as soon as they arrive at the client. This provides a level of copyright protection that straight download of video and audio files does not. Keep in mind that streaming is not a secure method of protecting your video and audio copyright, but it does provide some protection.