What is marshaling? What is use of marshaling?

Marshaling is process of converting unmanaged data type to managed data type.
Let suppose you are using unmanaged code. Data type is different in different languages.
For example int data type in C# is Int32 and it is 32 bit.
          But windows API has INT as integer data type. when we marshal Int32 to INT then we just change the type not data.