A function is a relation where an independent variable corresponds with exactly one dependent variable and passes the vertical line test.

A relation is where an independent variable can correspond with more than one dependent variable and does not pass the vertical line test.

In my graph, Each burger at McDonald's has their own price, but some burgers are also priced as a dollar via the Dollar Menu we all know and love. So now there are 3 - 4 burgers that are entirely different but they all cost one dollar. The independent variables, also called the domain, are the price of burger. The dependent variables, also called the range, are the type of burger.