A set is a collection of unique objects called elements. A set is usually written within curly brackets; the order of the elements in the set does not matter. $$\begin{array}{c}\text{List: }\text{apple},\text{banana},\text{bicycle},\text{peanut}\\ \downarrow \\ \text{Set: }\{\text{banana},\text{apple},\text{peanut},\text{bicycle}\}\end{array}$$ The example above is a list of objects, but sets can be made up of numbers as well! $$\begin{array}{c}\text{List: }3,3,2,1\\ \downarrow \\ \text{Set: }\{1,2,3\}\end{array}$$ Notice that although $3$ occurs twice in the list, unique elements of a set are only considered once and the original order of the list can be disregarded. Generally speaking, a number set is a group of numbers that share certain characteristics. $$\begin{array}{rl}\text{Even: }& \{2,4,6,8,10\dots \}\\ \text{Odd: }& \{1,5,7,9,11\dots \}\\ \text{Prime: }& \{2,3,5,7,11\dots \}\end{array}$$ When discussing mathematical theory, it is important to understand the distinction between certain number sets.