Semantic web is basically the idea of adding some meta-information (usually tags) to the information presented on the web in order to facilitate automatic processing by hinting the computers about what that information is about. For example, the labels that I try to put to my posts that you can see of the left hand side of this blog. The big problem with this approach is the effort needed to tag all the information on the web, that's why there is also a big interest in processing unstructured data, which recently lead to a spectacular Jeopardy show won by a machine build by IBM.
Comming back to radio, this principle of meta data first occured a few years ago when appart from the audio signal the broadcasters added a station id meta data that was used by the radios in the cars to automatically discover the local frequency of an FM radio. This was a nice feature cause once you have your presets in your car then anywhere you would go the radio player will automatically identify the local frequency of that radio based on the radio id metadata.
But a few weeks ago one of my favourite radios (Radio Guerrilla) introduced an even more spectacular feature. They added metadata for the song currently played on the radio, so on the tiny screen in my car I could see the name of the song and singer while playing. I wouldn't care much about this feature on a classic rock radio, cause there I know most of the songs they play, but Gurrilla plays many new songs, especially British and some Romanians ones so before this metadata I was always trying to memorize some lyrics from the songs I liked in order to search for them and identify the song. Now I have right in front of me on the radio display. This will mean I will probably not know the lyrics of the songs anymore :)