A variable is something that may change in value.
A variable might be the number of students in a class, number of miles you drive in a car, or age of a person.
A constant on the other hand, is something that has a fixed value. Within a computer program, a variable that is declared as a constant does NOT change its value at all during the execution of that program.
An example of a Constant could be the value of Pi. You want to keep the value to 3.14 no matter what.
Different languages have different ways of expressing if a variable needs to be a constant.
In Java, you would declare it using "private static final" keyword
private static final float PI = 3.14; private static final PERCENTAGE = '%'