Class diagram is an important one to know if you are a business analyst. Like use case diagrams, these class diagrams also explain the application in a pictorial representation, but in more technical way where a common user cannot understand by looking at these. A class diagram represents the relation between each class of the entire application. In one way it is a static representation of a structure of an application. It clearly tells the different classes and its attributes and relation between each class.
Normally for each member there will be few notations to tell more about the class member. These notations are placed before class. They are PUBLIC, PROTECTED, PRIVATE & PACKAGE.
In a nut shell, class diagrams can be defined as object oriented analysis and design documents. These class diagrams explain the relation between each class in a system and they properties, attributes, etc..
A typical class diagram will have three sections to represent each class.. In the below example I will explain one such class for your understanding.
In the above class diagram example, Customer is one class. And as each class is divided into 3 sections, under customer, there are attributes and under that there are methods. And there is a relation between each class.
A class is basically defined as a collection of objects that have the same structure, common behavior, relationship and semantics.
Alternatively defined as an instance of an object. We can find them by examining them in sequence and coloboration diagrams. They are represented in UML in rectangles.