Interfaces introduce another category of data types. (They are one of the few categories of types that don't extend System.Object.) Unlike classes, however, interfaces can never be instantiated. An interface instance is accessible only via a reference to an object that implements the interface. It is not possible to use the new operator with an interface; therefore, interfaces cannot contain any constructors or finalizers. Furthermore, static members are not allowed on interfaces.
Interfaces are closer to abstract classes, sharing such features as the lack of instantiation capability. Table 7.1 lists additional comparisons.
Given that abstract classes and interfaces have their own sets of advantages and disadvantages, you must make a cost-benefit decision based on the comparisons in Table 7.1 in order to make the right choice.