What is a Class

A class describes all the attributes of objects, as well as the methods that implement the behavior of member objects. It’s a comprehensive data type which represents a blue print of objects. It’s a template of object.

A class is simply a representation of a type of object; think of it as a blueprint that describes the object. Just as a single blueprint can be used to build multiple buildings, a class can be used to create multiple copies of an object.

A class is the core of any modern Object Oriented Programming language such as C#; most of the time, you will be writing classes. If you have a background in C or C++, many class aspects are similar, but some are different. For example, C# doesn't permit functions to exist outside the class declaration like C++; also, C# use the .NET Framework Class Library, so you have access to thousands more functions than the class libraries available to C++.

More detail about class please knock MSDN.