1. Natural
   1. denoted by N
   2. N={1,2,3,4... +∞}
2. Integers
   1. denoted by Z
   2. Z={-∞...-3,-2,-1,0,1,2,3...+∞}
   3. N is subset of Z: N⊂ Z or Z contains N: Z⊃N
3. Rational
   1. denoted by Q
   2. Q={a/b where a,b ∈ Z and b ≠ 0}
   3. Z⊂Q (Z is a subset of Q) or Q⊃Z (Q contains Z)
   4. A rational number is either terminating or repeating (have period) decimal
4. Irrational
   1. Any real number that is not a rational number
   2. Best known irrational numbers are: π, e and √2
5. Real
   1. Rational numbers + Irrational numbers
6. Complex
   1. denoted by C
   2. C⊃R
   3. Real numbers and Imaginary numbers