Using Interfaces is part of the Object Oriented Programming paradigm. This is not easy to explain and if you did not understand all those books I doubt anyone here can fully explain it in a manner that you understand. You may simply be missing some necessary basic knowledge. I suggest you get some beginners books on programming and Object Oriented Design.
A good place to start is CodeProject though! There are quite some beginner articles. One that helped me out when I was just starting is
How I explained OOD to my wife[
^]. Another one, by the same author, is a follow-up and also a great help to understanding abstract concepts such as Interfaces:
How I explained Design Patterns to my wife: Part 1[
^]
Edit:
Another great introduction article on Object Oriented Design:
Introduction to Object Oriented Programming Concepts (OOP) and More[
^]
Now the Entity Framework is a whole different story. If you have build 10 websites, as you say, certainly you have used DataAdapters, CommandObjects, ConnectionObjects, DataTables, DataSets etc. to read and write data to your database. This can be a tiresome process and DataSets and DataTables are not type-safe. This means every value is an Object, which can result in wrong TypeCasts etc.
The Entity Framework takes a lot of that work from your hands (like any Object Relational Mapper or ORM)! It creates your Classes based on your database's metadata. So if you have a table Customer with the columns ID, Name, Address and VatNumber then the Entity Framework can generate a Customer Class with those columns as Properties and their correct .NET types (string, int, long etc.). However, this is not easy. So I suggest you start reading some beginner articles here too. I have found
MSDN[
^] to be a great resource. But I am sure you will find many great articles and resources by using Google :)
Hope it helps!
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