Use of interfaces practical and real world example

Question

I've been attempting to figure out what interfaces are, and in principle, I've absorbed the term rather well. However, when it comes to actually employing them, I have some concerns.

The following is how most resources define interface:

"An interface is a contract that exists between the interface and any class that implements it." Any class that implements the interface must implement the interface's properties, methods, and/or events, according to this contract. There is no implementation in an interface; only the signatures of the functionality it provides. Method signatures, attributes, indexers, and events can all be found in an interface."

This is really clear, but my issue is: if interfaces are (according to this definition) a kind of blueprint or contract between themselves and classes, what happens if I define this interface, for example?

interface ITest {
    int SomeTestVariable { set; get;}
    int SomeTestMethod ();    
}

Create a class that implements both the interface and all of its methods.

class Test: ITest {
    int SomeTestvariable { set; get;}
    int SomeTestMethod () {
        return 1;
    }   
}

I remove it once all of the methods and attributes have been implemented.

class Test {
    int SomeTestvariable { set; get;}
    int SomeTestMethod () {
        return 1;
    }   
}

Now I need to find a class that's used this blueprint or contract. So, what's the difference between drawing this blueprint on paper and creating an interface?