Bicycle Tutor

Hey there,
this may seem a bit of a dumb question. I'm after buying a new rear wheel, I've built one up at Chain Reaction Cycles, but there's one issue I'm not sure about.
Are all axle lengths universal? I'm asking this because looking at Quick Releases skewers and axles on different sites they don't mention the lengths of them, but surely not all bikes have the same space at the back.
So are there big differences in axle sizes or does it not really matter? The wheel's costing me £72 (7spd cassette included) so don't wanna get it and see it won't fit in my frame.
Thanks for any help in advance

Yes, the frame spacing is quite different, depending on age and usage of the bike. Modern road bikes: 130mm, mountain bikes 135mm, downhill bikes more, old mtbs: 130mm, old road bikes (up to 6 gears inthe back, I believe): 126 or even less. You want to make sure the O.L.D. (over locknut dimension) of the hub's axle matches your frame spacing.
List with everything:
http://sheldonbrown.com/frame-spacing.html
Good luck with the wheels!