This document discusses relations and functions in mathematics. It provides definitions of both relations and functions, noting that a function is a special type of relation where each input is mapped to only one output. The key differences between relations and functions are explained, including that relations can have a one-to-many or many-to-one relationship while functions must have a one-to-one relationship. Examples of graphs of functions and relations are given, showing that a vertical line test will pass through a function graph only once but may pass through a relation graph multiple times.