Interface and implementations

Hello, I am quite new to Java and I`ve encountered "interface"/"implements". I can`t seem to understand the idea in these, it`s like adding a modules to a device implementing many interfaces with their modules. I think you can do the same with static methods and abstract classes or it`s different approach? Is it often to see using interfaces and implementing them into a class? Since you can add many, what is the approach - separate them in different java files or just stick in the same file?

Look at the Collections framework for an example of interfaces in action.