Can someone please tell me what can interface is?

Question

On the question sheet I found in my brothers stuff there was a question about completing some interface code, but I've got no idea what an interface actually is, as near as I can tell its a list of methods that a class has. So is it like a contents page of methods that you can call?
Also, here is the question on the sheet.
5. Consider the following Java Library interface
public interface Comparable<T>
int compareTo(T rhs);
Complete the implementation of the class below that implements the above interface. The compareTo method should return -1 if value is less than rhs.value, 0 if both sides are equal and +1 if value is greater than rhs.value.
public class MyInt implements Comparable<MyInt>
private int value;
MyInt(int x) {...}
public String toString() {...}
public int intValue() {...}
public int compareTo(MyInt rhs){...}
public boolean equals(Object rhs) {...}
In the last 2 weeks I've learnt some much about java just from answering these questions on this work sheet. I'm starting to feel that I actually understand what to do and I'm enjoying it a lot more now, Before I was just throwing lines of code at the compiler and hoping for the best.

Answer 1

http://java.sun.com/docs/books/tutorial/java/IandI/index.html