>>7
Naturals, integers, rationals are countable (N,Z,Q).
Reals are uncountable (R).
You can get to higher ordinals by using the power set operation.
To show that there is a difference between uncountable and countable sets, you should look up Cantor's diagonalization argument which shows that the set of reals has a greater cardinality than the set of integers (and a generalized version of it can be used to show that for power set operations).
Countable could also be considered as
enumerable. For example, for natural numbers, you can start with an initial element (0), take a successor function which when applied to a number, produces its "successor" (also a natural). If the successor of 2 numbers are equal, the numbers are equal (this is how you define the successor function, as an axiom; it's usually s(x)=x+1 if you want a more familiar form). Also, 0 is never the result of a successor function's application (initial element). This may make it seem that the set of naturals takes the shape of {0,s(0),s(s(0)),s(s(s(0))), ...}, and is an countable/enumerable infinite set. Now to show that this set contains all naturals, you either need an induction schema axiom (such as with Peano Arithmetic) or an axioms like this (in the weaker Robinson Arithmetic):
Given a natural y, it's either 0 or it's the result of the successor function (
y=0 ∨ ∃x (Sx = y)). This axiom would be a theorem if an induction axiom schema was present.
Induction axioms can have many forms, but here's 2 of them:
Given a predicate P(n). If P(0) is true and P(k) implies P(s(k)) then P(n) is true for all natural numbers n. At times this axiom seems a self-evident truth as if you know P(0) is true and P(k)->P(s(p)) (where s is as defined previously(successor or
1+)), we could see that P(0),P(1),P(2), ... and so on will all be true, but without the axiom we can't say that for
any n it is true, hence this is where the countable infinity truly appears (and it's also the cause of the whole consistency problem - or Godel's incompleteness theorem).
A different way of stating the induction axiom would be like this: A is a set. 0 is in A, for any natural number n, if n is included in A then s(n) is in A, then A contains all natural numbers.
One could go on here about isomorphisms between countable sets and models, but that would be me getting carried away more than I should.