I'm not sure what you mean when you say 'Class Form'. Are you referring to the form class? E.g. the class used to create Forms in winForms programs?
A class diagram is a text/graphical display of a class, outlining various aspects of its structure such as name, parameter types, included methods, etc.
Diagrams incorporating several classes are uses to show how software components fit together and interact.
A picture would explain it better. google images for class diagram and you'll find plenty of simple examples.