This is literally the building blocks of any object oriented programming-based application. You have probably been drawing class diagrams since your first year (unless you are a first year reading this, then good for you!).
The design of your classes are vitally important as they, along with your use cases, will be used as the basis for your Interaction Sequence Diagrams. Classes will also be used as a type of blueprint for your database design (Entity Relationship Diagrams).
Class Diagrams show the classes that you will be implementing in your system. Within classes there are attributes and operations, each with their own properties. Finally you will also model the relationship (or, actually, associations) between classes.