OOP (a.k.a. Object Oriented Programming) is the idea that data in a computer's memory can be regarded (and acted upon) as a theoretical object. A good example of this is a hot dog stand. In reality, each hot dog stand has its own statistics (temperature of the oven, number of hot dogs available, etc.). However, from and Object-Oriented standpoint, every statistic can be treated as a property of the HotDogStand object.

See also:
C++
Java