This is a feature of 2005,when we have large and unwildly 
classes instead of writing all the methods in same  program 
we will  divide the class into number of programs with the 
same class name,these classes are preceeded with partial 
keyword,when we compile all this partial classes only one 
dll with the same class name will be created. 


class with some 20 methods:
Example.cs
class classA{
m1( );
m2( );
.
.
.
m20( );
}

Apply partial concept:

Example1.cs
partial class classA
{
m1( );
m2( );
.
.
m8( );
}
example2.cs
partial class classA
{
m9( );
m10( );
.
.
m15( );
}
Example3.cs
partial class classA
{
m16( );
.
.
m20( );
}

if we compile Examle1.cs,Example2.cs,Example3.cs it will 
create only one dll 

“partial” keyword can be used to split a single class, 
interface, or struct into multiple, separate files in 
ASP.NET (using C# as the language). This permits multiple 
developers to works on these files, which can later be 
treated as a single unit by the compiler at the time of 
compilation and compiled into MSIL code. 

1.Partial classes can improve code readability and 
maintainability by providing a powerful way to extend the 
behavior of a class and attach functionality to it. 

2.The partial modifier can only appear immediately before 
the keywords class, struct, or interface.

3.Nested partial types are allowed in partial-type 
definitions.

4.Using partial classes helps to split your class 
definition into multiple physical files.

5.Note that the partial keyword applies to classes, 
structs, and interfaces, but not enums