API vs SDK

http://stackoverflow.com/questions/834763/api-vs-sdk
API could be said to be the "declaration" and the SDK the "declaration + implementation(definition)" of an interface(method/technique to drive/use something)

Note that in OOP, the interface is usually a virtual class (C++), or an interface (Java). People can do 2 things with such a class/interface.

1. If what the programmer is provided is just an API with no accompanying implementations - he/she can implement the interface aka create a derived class and use the base class' pointer to call the functions defined. 
2. If what the programmer is provided is an SDK, with the interface specified in a virtual class (or an "interface") then the libraries provided will implement the SDK... all the programmer needs to do is to instantiate the object of this interface (as specified in the documentation), and then use it.