They create irrational numbers with rational numbers set of real numbers.
The numbers are irrational: \[\sqrt{2},\ \sqrt{3},\ \sqrt{5},\ \sqrt{17},\ \sqrt [3]{2},\ \pi \] None of these numbers can be written in the form of a fraction.
Note ! Not every root is an irrational number, eg .: \[\sqrt{9}= 3 =\frac{3}{1}\]