i IS a number, if by 'i' you mean "the (square) root of minus one". It's what you call an imaginary number.
What you call "numbers", are real numbers (6, 85, 9001, 1337), numbers such as i, -7i, 49i are imaginary numbers.
"real numbers" + "imaginary numbers" = "complex numbers", such as 1337 + 9001i, or 6 - 3i
In total, there are:
natural numbers,
integers,
rational numbers,
algebraic numbers,
real numbers,
complex numbers,
p-adic numbers,
surreal numbers, and
hyperreal numbers.
Natural numbers can be defined as either "positive numbers", or "non-negative numbers" -- note the difference between positive numbers and non-negative, is that positive numbers do not inlude '0', however sometimes 0 is referred to as a natural number
Integers are simply "whole numbers" (i.e. not fractions), 1337, 9001, 666, are integers
A rational number is any number that can be expressed as a fraction, e.g. 0.5 can be expessed as 1/2, 0.33333 can be expressed as 1/3
An algebraic number is a number that is a root of a non-zero polynomial in one variable with rational (or equivalently, integer) coefficients, for example
and
are algebraic since they are the roots of polynomials
x[sup]2[/sup] − 2 and 8
x[sup]3[/sup] − 3, respectively.
Er... read the rest:
http://en.wikipedia.org/wiki/Number_system