C# Interfaces. Why are they separate? And why would anyone use this?

Hi All,

I have a question about C# interfaces.  Basically I understand that when using interfaces it means you're building loosely coupled applications. The interface is merely a declaration.  All members are public by definition.

Here is my problem / confusion.

Why do we have separate interfaces and separate classes?  Why can't I define the interface within the class?
This is where I am confused. I think I am not understand the interface concept properly....

Kindly help me to some clarity on this.

Thanks
Vijay r