Class diagram describes the relationships between classes. A Class is type of an instance object. Instance object of a specific class will follow the behaviors we have set for that class. That is OOP, baby! There are many types relationships that a class can have with other classes. These relationships are guarded by rules. Rule can be a set of standard rules or user specified rules. Lets talk about relationships guarded by set of standard rules: 1. Association 2. Aggregation (Association + more rules) 3. Compostion (Aggregation + more rules) Exaple of Association: 'a customer places an order.' customer is a type of object (class). order is another type of object place order is an 'action' that creates an relationship (link) betwen the two classes. Rules: 1. How many orders can a customer place? Rule says: 0 to infinite 1. Can an order be placed by how many customers? Rule says: 1 and only 1 [>:)'> Now here is the clasess with summarized rules: customer (1..1) -------place order ------- (0..infinite) order