Abstract vs interface

Interfaces are closely related to abstract classes that have all members abstract. a. For an abstract class, at least one method of the class must be an abstract method that means it may have concrete methods. b. For an interface, all the methods must be abstract c. Class that implements an interface much provide concrete implementation of all the methods definition in an interface or else must be declare an abstract class  In C#, multiple inheritance is possible only through implementation of multiple interfaces. Abstract class can only be derived once. d. An interface defines a contract and can only contains four entities viz methods, properties, events and indexes. An interface thus cannot contain constants, fields, operators, constructors, destructors, static constructors, or types. e. Also an interface cannot contain static members of any kind. The modifiers abstract, public, protected, internal, private, virtual, override is disallowed, as they make no sense in this context. f. Class members that implement the interface members must be publicly accessible.

additional to what Kirtan said, the idea of Abstract is also to make you create default Operations on the basis, like for example, you give a public that is the Method that can be overriden, and it runs a private to create the right string to access the db or other services... or so on