All programs require one or more classes that act as a model for the world.

For example, a program to track student test scores might have Student, Course, and Grade classes. Our real-world concerns, students and their grades, are inside the program as classes.

We represent each student as an instance, or object, of the Student class.

This is object-oriented programming because programs are built around objects and their interactions. An object contains state and behavior.

Classes are a blueprint for objects. Blueprints detail the general structure. For example, all students have an ID, all courses can enroll a student, etc.

An instance is the thing itself. This student has an ID of 42, this course enrolled that student, etc.

Let’s review with another example, a savings account at a bank.

What should a savings account know?

• The balance of money available.

What should a savings account do?

• Deposit money.
• Withdraw money.

Imagine two people have accounts that are instances of the SavingsAccount class. They share behavior (how they deposit and withdraw) but have individual state (their balances), and even with the same balance amount these accounts are separate entities.