Have a look at the manual. It's slightly out of date but should still answer your questions:
http://downloads.omnigroup.com/softw...1.5-Manual.pdf
For further reading, see the "OmniFocus, GTD, and You" white paper:
http://downloads.omnigroup.com/softw...dOmniFocus.pdf
You may also want to check out the video tutorials at:
http://www.omnigroup.com/products/omnifocus/videos/
Hope this helps.
-Dennis