Formulas are a quick and easy way to get useful and meaningful data calculated, validated and displayed in Salesforce. The online help has all of this information (plus a lot more), but these tip sheets are very handy for a reference overview in a nice neat PDF. It also gives you a good feel of what you can do and what Operators and Functions area available to you.
Formula Quick Reference:
Examples of Formulas:
These are available from the Tip Sheets page at the Salesforce Learning Center, take a look around, there are lots of great resources listed.