Apple’s Knowledge Base article lets you learn about the concept of permissions (or “privileges”) in Mac OS X, issues that can arise due to incorrect permissions settings, and how to troubleshoot them. Be sure to read the sections called Permissions Defined and Issues Related to Permissions even if some of the other sections seem too technical. This is in fact an article that any user of Mac OS X should at least skim.
Probably actually a better thing to do would be to read Take Control of Users and Accounts in Panther (US$5, PDF, 65 pages), but I haven’t finished reading my copy yet and can’t offered a detailed critique. The Take Control books are excellent and more than worth their trivial price.