A Rational number is a real number that can be written as a a simple fraction, a ratio of two integers . An Irrational number is a real number that cannot be written as a simple fraction.

For example, 1.5 is a rational number because it can be written as 3/2; 101 is a rational number because it can be written as 101/1, or 202/2. But pi=3.14159... cannot be written as a simple fraction and therefore is conconsidered as irrational.