PT's response is a good way of thinking about it as a shorthand.
The derailleur cage length determines how much slack (known as capacity) the derailleur can take up as you change gears. It's worked out by the following formula:
Gear Capacity, measured in teeth = (largest rear sprocket + largest front ring) - (smallest rear + smallest front)
So if you had an 11-27 cassette, and a 50-34 chainset, your gear range capacity would be
(27+50) - (11+34) = 32
Provided the derailleur's capacity handling is higher than the calculated number it'll work, so long as it can also handle the largest rear sprocket, and also sometimes manufacturers also stipulate a max difference between the inner and outer front rings.
Road bike derailleurs have a smaller capacity and max sprocket size than MTB ones, which is why you sometimes see MTB rear mechs on really hardcore, low-geared road tourers.
Long cages give a bigger capacity but less crisp shifting, in general terms.
I run a 50-34 up front and an 11-28 on the back. That right at the end of the short cage Shimano 105 RD (max capacity 33T). The longer cage version handles 39T
Manufacturer's spec should tell you what you need to know.
Bet you never asked now
Edited by MikeP at 22:34:52 08-01-2013
PSN ID: Mikenetic