Right, where to start...
The
BBC was originaly a radio broadcaster, until 1929, when they experimented with the first Television broadcasts, culminating in them starting regular broadcasting in 1932. This was the beginings of BBC1 (the UK's first TV channel).
ITV (Independant TeleVision) (our second channel) began broadcasting in 1955, as a direct competitor to the BBC. The BBC's answer was a third channel (BBC2) which started broadcasting in 1964. Our fourth channel (
Channel 4) started in 1982. These were our only television channels until
British Satellite Broadcasting started in 1986, eventualy merging with
Sky Television in 1990 to become
British Sky Broadcasting. This is when satellite broadcasting really took off in the UK. The only other notable channel is
Five, which started in 1995.
When digital satellite took off around 2001, the BBC, ITV, Channel 4 & Five started up many new channels dedicated to certain types of programming. They handn't been able to do this before, as there was only so much room on the analogue TV network for channels.
Basicly, BBC1 & 2, ITV, Channel 4 and Five are what is known as 'Terrestrial' channels, ie those able to be picked up with a basic television using a rooftop aerial. Any other channels use special set-top boxes to recieve them (Sky's satellite reciever, NTL/Telewests cable TV reciever, or Freeviews terrestrial reciever (which picks up a digital signal via the roof aerial)). Not all people in the UK currently recieve digital TV, so only about half of the country (at a guess) can recieve BBC3.
Sorry if that was a bit long-winded. I got carried away whilst typing!