When you write a program in an object-oriented language, you don't define actual objects. You define classes of objects, where a class is a template for multiple objects with similar features. Classes embody all the features of a particular set of objects. For example, you might have a Tree class that describes the features of all trees (has leaves and roots, grows, creates chlorophyll). The Tree class as an abstract model for the concept of a tree -- to reach out and grab or interact with, or cut down a tree you have to have a concrete instance of that tree. Of course, once you have a tree class, you can create lots of different instances of that tree, and each different tree instance can have different features (short, tall, bushy, drops leaves in autumn), while still behaving like and being immediately recognizable as a tree.
Laura Lemay, Object-Oriented Programming and Java
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