The simplest numbers are the natural numbers, i.e. 0, 1, 2, 3, 4, 5, etc. They are yielded by counting. Especially the number 0 is to indicate that there is nothing.
In order to do the subtraction between a smaller natural number and a greater one, the negative numbers -1, -2, -3, etc. are introduced. They and the natural numbers are called the integer numbers, which are defined as difference between two natural numbers.
Among the integer numbers, the division is not always possible. Therefore they are extended by the rational numbers, which are defined as quotients of integer numbers. Numbers yielded in practice, e.g. by measurements, are also rational numbers in form of decimal numbers – which are not always simple and perfect quotients.
The very first irrational numbers are the square roots, the cubic roots and the number π, which are yielded by geometrical means. The square roots, the cubic roots as well as the other numbers composed of roots are special cases of numbers, called the algebraic numbers, which are zero points of polynomials with rational coefficients (however not every algebraic number can be stated as an expression with roots). The number π does not belong to this group, as well as many numbers yielded later by analytical means (exponents, logarithms, harmonic functions etc. and integrations). They are called the transcendental numbers.
With the assumption of the continuum model, one can fill in completely to the rational numbers and get the real numbers. There are some different definitions of real numbers. In one way a real number is considered to be the limit of a sequence of rational numbers, with the assumption that endless processes (sequences) can be done (idea of Cauchy). In another way, a real number is considered as a cut (idea of Dedekind). There are also other definitions. One important meaning of the real numbers is that the length of every segment on a straight line can be exactly measured with the real numbers.
A remained problem is that not every polynomial with real coefficients has solutions. So the real numbers are extended to complex numbers, with which this problem is solved.
Note that there is (are) also extension(s) of the complex numbers, but with different characteristics.