Magic Number,
Yes mate pretty much what you said. A double (in football terms anyway) is where you predict the winners of two events (i.e: Matches or Leagues or Cups etc). So at the weekend I placed £50 on both Newcastle & Birmingham to win.
If one of them fails to win the bet is down. If they both win, your in the money.
Can't remember off hand but I think the odds were just under 3/1 so, if Newcastle had of kept a hold of their 1-0 lead, I would have won about (well just under) £150 and got my original £50 back so, about two hundred quid back for my fifty quid stake.
There is no real minimum so you could have put 30p on it if you wanted and returned about £1.20 and the maximum stake allowed with my bookie is £300 which would have returned just under £1200 inc your stake.
Some people will bet on other doubles such as:-
Chelsea to win the league and Cardiff to win the Championship for example.
Newcastle to win the F.A Cup and Liverpool to win the league cup etc etc
Hope this helps.
I'm a bit of a regular gambler so if I can help explain any other bets then please shout up.
Cheers
Riches.