Before cable and sky all TV signals came to your TV via the aerial. These signals were analogue signals rather than digital.
Cable TV comes into your house via a cable in the ground (surprise surprise). Down the same cable can come your telephone and broadband.
This is supplied by either NTL or Telewest (now one company) but if you have not got the cable in your street you cannot get it.
Sky TV comes to your house from a satellite in the sky to a dish on the side of your house (or maybe the front or back).
Both Cable and Sky now distribute their TV as digital.
The other way to get digital TV is to buy a digi box (or freeview box).
This is a cheap way to get digital TV as you buy the box, plug your aerial in the back and away you go. Note that not all areas can get the freeview signal.
In a few years time ALL analogue TV will be switched off, so you will HAVE to get digital TV.
By then most TVs will probably have a freeview box built in so it will not be a big problem, but old TVs will not work unless you buy an additional digi box.