Let me set the context. In the 1960s British broadcasting was monopolised by the BBC. Because the BBC was run by a load of old fuddy-duddies (allegedly) who were out of touch with the population (also allegedly), pop music was limited to a few hours per week.

Along came an Irishman named Ronan O'Rahilly. Amongst other things, he was a music producer and wanted to get his artists some radio play. This was difficult as a) the BBC didn't play pop music and b) Radio Luxembourg would only play pop music if the record company paid for it. So he decided to start his own radio station. You can imagine how the conversation with the broadcasting authorities might have gone, if there had been one.

"I want to start a new radio station"
"Jolly good. What's the output going to be?"
"Pop music"
"I think you'll find that that market is already covered by the BBC. They broadcast, oh, almost an hour of pop music every Saturday afternoon. What else are you going to do?"
"Nothing. Just pop music"
"I can't see that catching on. I don't think we can give you a broadcasting licence"

So Ronan decided to work outside the law. He bought a ship, converted it to a radio station, parked it in the North Sea just outside territorial waters and started broadcasting. Pirate Radio ships were born.

Radio Caroline was the first; soon there were several others. After a few years the law was changed (and BBC Radio 1 and 2 were created**). It became illegal for the pirate ships to be supplied from the UK. Most of the stations closed down, but Radio Caroline carried on broadcasting, on and off, until the ship sank in 1980.

But in 1983 it came back with a new ship. I used to listen to it quite a bit in my teenage years (as Radio 1 was rubbish, even then). It broadcast on an AM frequency so the quality wasn't great and it would fade in and out a lot. The records jumped from time to time if the water was choppy, and sometimes it would go off air for several days at a time when there was a storm.

Today Radio Caroline still broadcasts (legally) via the internet, and occasionally from events, etc. on a restricted licence.

* Radio Luxembourg beamed English language pop music programmes to the UK in the evenings (possibly because AM transmissions travel further when it is dark).