Wolfram Research
Wolfram Math World: Rectangular Hyperbola
With this math site you will find the distinguishing characteristics of a rectangular hyperbola and other frequently used terms when referring to this special hyperbola. This site also gives formulas, pictures, and links.
Wikimedia
Wikipedia: Hyperbola
This site from the Wikipedia encyclopedia has formulas and examples of hyperbolas. A detailed graph and several formulas and examples are given along with several links to additional information on related subjects.