Cardinality and Ordinality
Quantity and order
There are to primary aspects of numbers that are crucial for a complete understanding of numbers. These are cardinality and ordinality (or simply quantity and order.). Cardinality is to say the amount in a given set. For example if i have a group of apples, how many apples are there. Ordinality on the other hand, are numbers on a line or in a sequence (eg. 1,2,3,4,5,6,7,8,9,...). Both probably seem as rather easy concepts, but as a number line isn't a logarithmic scale it can sometimes be hard to link the two concepts.
I suggest the book "Here's Looking at Euclid" if you find this interesting.
