Not sure I can explain it very well but it's the highest (and I think the oldest) honour in the British honours system (it's higher than the ordinary knighthood, which is what Paul McCartney has). It's the only one that the Queen decides who receives it - all other honours are suggested & decided upon by the government at the time & the Queen pretty much just has to go along with whoever they choose each year while this one she decides who gets it & the government have no say in it - plus there is only a certain number allowed at any one time, 24 I think. Not sure what a Knight of the Garter title before their name is but I know it's not Sir...might just be Knight.
Here's the wiki page as just realised that'll probably help more & not veer this thread way off topic
http://en.wikipedia.org/wiki/Order_of_the_Garter