http://en.wikipedia.org/wiki/Bicycle_wheel
http://www.parktool.com/blog/repair-help/categories/hub-service
http://en.wikibooks.org/wiki/Bicycles/Maintenance_and_Repair/Hubs/Cleaning_and_repacking_a_hub
Harris: http://sheldonbrown.com/harris/hub-parts.html
--
Sheldon Brown
Cassettes & freehubs: http://sheldonbrown.com/k7.html
Hub overhaul: http://sheldonbrown.com/tooltips/hubs.html
Spacing: http://sheldonbrown.com/gloss_sp-ss.html#spacing overlocknut distance
--
From Sheldon Brown
6- and 7-speed hubs generally use 126 mm spacing, while 8-/9-speed hubs use 130 mm (road) or 135 mm (MTB/hybrid).
Note, also that most rear derailers do not care how many gears you have. You do not need to have a so-called "10-speed" derailer to use a 10-speed cassette.
You also can do a minor modification to a traditional Deore MTB rear derailer so it will work even with a "Dyna-Sys" series cassette that has a 36-tooth large sprocket.
Common spacings
100mm = Modern front hubs
126mm = Rear 6 and 7 speed (road)
130mm = Rear 7-speed (MTB) and 8- 9- and 10-speed (road)
135mm = Rear 7- 8- and 9-speed (MTB)
--
RANDO'S Hubs: Shimano Deore DX FH-M650 [VeloBase]: Rear spacing 130-135mm; low flange ("low dish"); spoke drilling 32, 36. (1990-93)