I find lots of useful instructions at the park tool website. Also if you have a shimano derailleur check out the techdocs section on shimano website. Most derailleurs have the same adjustments. Those High and Low limit screws push up against the body of the unit to physically stop it from moving further across the cassette/freewheel.

http://www.parktool.com/blog/repair-...nts-derailleur
http://techdocs.shimano.com/techdocs/index.jsp