A class represents some concept, physical or otherwise, in the system being modeled. Classes describe sets of things or concepts that have similar attributes, behavior and relationships. They are a higher order concept to objects, which represent instances of the class.
Explanation
The class is one of the most universal of the UML model elements. It is the more general description or definition of the set to which objects (class instances) belong. It is from these objects that the term object oriented is derived so it is often central to the modeling endeavor. We use the word in our natural language when we ask questions like "what class of thing is it"? In these circumstances we want more general information about the specific thing and want to understand what type it is. A class is represented by a rectangle that has a number of compartments. There are three standard compartments, but other compartments may be used to show additional information. The three standard compartments are the name, attribute and operation compartments.
An object or class instance is an instance of a class. It represents an actual example or instance of a class. While the class is a generic description of a set of things with related properties and behavior, the object is one of the members of the set.
An association class is related to a class in so far as it is has both association and class characteristics. The association class can always be distinguished because it is attached to an association by a dashed line.
