Any number which
can be expressed in the form p/q where p and q are positive or negative
whole numbers is called rational.

For example

2/3, 3/5, -5/7, 49/-56 are rational numbers

The natural numbers
and integers are subsets of the rational numbers, which is easily
seen by putting q = 1

Any terminating or recurring decimal can be written as a rational
number

For example, 37.2567 = 372567/1000099 and 0.1313131313... = 13/99

But numbers like √2,
and √3 and
π cannot be expressed as rational numbers
and are called irrational.