Since the idea of the object is so central, a full discussion of what objects actually are is essential to understanding object-oriented systems and their needs, particularly from the perspective of the user interface. What we programmers call classes are really classes of objects. That is, a set of objects are of the same class if they have the same properties. This usage is just English, and is really the correct way to think about things. The prime directive of object-oriented design is data abstraction.
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A given object doesn't have any idea of what the innards of other objects look like, any more than you might know what your spouse's gall bladder looks like. You may have read in a book somewhere that an object is a datastructure of some sort combined with a set of functions, called methods, that manipulate that datastructure. First and foremost, an object is a collection of capabilities. The data is irrelevant. Most object-oriented designers will spend weeks, if not months, in design before they even think about the data component of an object.