Where will the interface is used in a project?

I am not sure, if this is the right question to be asked here. But I have checked number of places but didn't get the perfect answer of these.

I have been hearing these question for long in all the interview questions portal. All they explain is the difference.

I am NOT looking for the tabular differences, but what is the most appropriate place to make use of them.

NOT looking for differences

• Suppose, you are developing a Web based project. So in what scenario you will use interface or abstract class.
• Why should I use Design patterns
• If EF is slower than native SQL, why should I use EF.

Please don't relate this question with any specific technology, as these things can be found in any programming language.

Looking for some precise answers.