A noun is a thing, e.g. cat, room, exam. Most of the time, we can talk about how many of these things we have: Bobby has three cats at home! It’s amazing how many rooms are empty in our school. I have ...

For more information about countable and uncountable sets, see books about "Analysis" (as advanced calculus is called). For example, Introductory Real Analysis, by A.N. Kolmogorov and S.V. Fomin (see ...