NumberPhile made a video on this.
There are many types of infinity, and the two main categories are Countable and Uncountable Infinity.
Countable Infinities are like the list of Whole Numbers, 0, 1, 2, et cetera, et cetera, or the list of Natural Numbers, 1, 2, et cetera, et cetera.
But what about Uncountable infinites?
Consider Rational Numbers (Positive and Negative Integers).
In order to start counting, you would have to go to the very beginning of the number line.
Which of course, is impossible.
Decimal Numbers.
Start from 0.
That's easy.
But then you have to find the next smallest number.
That would have infinite zeros before the one.
So no for Decimals.
Fractions?
Smallest fraction?
Start from 1/Infinity.
Nope, nope, and nope.
But this is much better explained in this video:
Have fun!
Countable Infinities are for example
N (
Natural Numbers, {1,2,3,...}, sometimes also containing the 0),
Z (
Integer Numbers, {...,-3,-2,-1,0,1,2,3,...}) and
Q (
Rational Numbers =
Z/
N).
A group of numbers is countable, if all members can be brought into definite order, where any number can be defined as the n-th number of that group.
N: Easy, since you can simply match N -> N (1 is the 1st number, 2 the 2nd number, ...).
Z: Easy with a small trick: 0,1,-1,2,-2,3,-3,4,-4,..., so -4 is the 9th number in the line.
Q: A bit trickier: Write a matrix with 0 in the top left corner. Expanding to the right in the top row follows
Z, expanding down in the leftmost column follows
N. The remaining fields are then filled in with the top number divided by the left number. This will construct all possible members of
Q (remember its definition as being
Z/
N). Now for counting the numbers you have to draw diagonals over this matrix from up right to down left. The first one only hits the 0 (first column, first row). The second one hits 1 (second column, first row) and 1 (first column, second row). The next one hits -1 (third column, first row), 1/1 (second column, second row), 2 (first column, third row). And so forth and so forth. Via this method every number has its definite place. You can expand the definition to remove doubles from the list. Easiest would be to never hit the first column except for the 0. This would keep numbers like 1, 1/1, 2/2 counted separately. You could also expand the definition, that only numbers are counted after being reduced completely which were not counted before (so 1/1 wouldn't be counted, since 1/1 = 1 and 1 had been counted beforehand). But no matter which definition you use, members of
Q can be counted, so
Q is part of the countable infinities.