Define ℕ.
Natural numbers: the set of positive whole numbers and zero.
Define ℤ.
Integer numbers: the set of whole numbers, including both positive and negative numbers including zero.
Define ℚ.
Rational numbers: the set of all numbers which can written as a fraction, including both positive and negative numbers including zero.
Define Irrational numbers.
Irrational numbers: cannot be written exactly as a fraction: surds, pi, Euler's number.
Define ℝ.
Real numbers: all possible real world numbers: irrational, rational ℚ, integers ℤ and natural ℕ.
Define Ordinal numbers.
Integer numbers used to describe numerical positions of objects in relation to each other: 1st, 2nd, 3rd.
Which set of numbers would you use for counting how many computer science topics are left?
ℕ - natural numbers.
Which set of numbers would you use for measuring the the length of a football field?
ℝ - real numbers.