An instance variable is a variable which is related to a single instance of a class. Each time an instance of a class is created, the system creates one copy of the instance variables related to that class.
An instance variable is delcared inside a class, but not within a method. A variable which is defined inside a class and a method is known as a local variable.
The opposite of an instance variable is a static variable, also referred to as a class variable. A static variable exists across instances of a class.
By default, all variables are created as instance variables.