Originally Posted by Al1943

A lot of incorrect information above. Various brands of derailleurs do not mix with other brands of shifters. There are significant differences.
The distance that a derailleur trevels with each shift depends on two variables cable pull (what the shifter does) and actuation ratio based on the derailleur's geometry. All modern Shimano rear derailleurs, except pre-9-speed Dura-Ace, have the same actuation ratio. This is why they are cross-compatible. Other brands have different actuation ratios and are not cross-compatible, some are close enough to work, sort-of.
Here is a link to good information about cog spacing, actuation ratios, and cable pull. Scroll down for the charts. Actuation ratios are shown as "pitch": http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946