What does it actually do?
What does proper adjustment look like?
from a previous post of mine, assume that you have a shimano derailleur.
By tightening the B-screw, it rotates the RD clockwise around its mount. You need to look at it from the side and see how far the pulley is from the largest cog, when the chain is on the largest cog. By tightening the screw it will move the RD away from the cog. You should not have to adjust this unless you change rear cogs (i.e. go to a bigger or smaller rear cog). If its not tight enough, then it will make a lot of gear noise.
The park website gives some good pics, try http://www.parktool.com/repair/readhowto.asp?id=64 about 1/2 way down the page.