A set is a collection of well defined things.
eg A set of polygons
eg A set of integers
We use capital letters to name a set, and { } brackets to indicate what items are in the set.
eg D = {1, 2, 3, 4, 5} Set D includes the whole numbers 1, 2, 3, 4 and 5.
A subset is part of a set.
eg E = {1, 2, 3} Set E is the first three items from Set D.
A null set is a set with no items.
eg F = { }
A finite set is a set with finite items, a fixed amount of items.
eg Set D is a finite set as it has a fixed number of items, 5.
An infinite set is a set with an infinite amount of items, no limit.
eg G = {positive integers} Positive integers = 1, 2, 3, 4, 5, 6, ... forever