interface/class

You mean usage. Wright?

Here's a very short expalation:

A Interface defines a contract that the
derived class should implement.

Meaning it defines the methods(their signatures) that a class and/or struct should implement.

Also C#/.Net allows for multiple Interface but single class inheritance.

Supose you have a LogSystem. That system should write something.

So:

interface IWrite
{
    public void Write();
}

The interface above defines a contract for the classes.It's contract it's a single method in this case that returns null.

Now you could have like a WriteToLog class, WriteToOutput(e.g. console),
to a DB.

but all of this classes Implement the IWrite interface.

public class WriteToLog: IWrite{
    public void Write(){
       //implement it here
    }
}

public class WriteToConsole: IWrite{
    public void Write(){
       //implement it here
    }
}