A standard format for representing an implementation of a class in the Java(TM) programming environment. Each file that is in class file format is a representation of a single class.
A class file is split into a number of sections including (but not limited to) a constant pool; the super class of this class; a list of the interfaces implemented by the class; a list of the fields within the class; a list of the methods; and a list of attributes associated with the class (this might for example contain debugging information) - MatthewCaswell, 31 Oct 1997
For more information on class file format, see also