An article explaining the prototype-based implementation of OOP, inheritance, and classes in JavaScript. Concepts are illustrated with numerous code samples. Netscape makes a special effort to explain JavaScript's OOP in terms that C and Java coders will understand.
Sun's Object-Oriented Programming Concepts (from The Java Tutorial)
Although intended as a primer for aspiring Java programmers, this site covers the basics of OOP in generalized terms and makes good background reading for OOP in any language.