Objects are instances of classes in CLOS. Classes, Methods, Generic Functions, ... are also instances of classes. Classes support multiple inheritance. Instances have an identity, slots and a class.

CLOS does not use methods attached to classes, which can be invoked by message sending or similar. Instead CLOS uses Generic Functions which consist of one or more Methods. Methods are dispatched over classes, using possibly multiple arguments. Generic Functions are themselves objects and they are defined outside of classes. When a Generic Function gets called, all the applicable Methods are combined to an Effective Method that then will be executed.