Linear Functions. They are represented by straight lines. They are given by equations such as

Their graphs are always represented by a straight line. You have to use a ruler to plot them. They look like this:

Quadratic Functions. They are represented by parabolas, which are curvy (not straight) lines sort of like cow horns. By the way, if you throw a rock, its line of fall is represented by a parabola (with horns obviously pointing down). Try it where windows and people are not in danger. Parabolas represent quadratic equations such as

Because parabolas are curvy, you cannot use a ruler to plot them. They look like this:

Absolute Values. In simple cases, they are represented by jagged lines composed of straigt segments. In more complicated cases, where the absolute value is mixed in with other functions, the lines may be not straight, however they have still some points where the graphs are not smooth and where the tangent angle changes abruptly. An example of a simple graph involving absolute value is

You can use a ruler to plot simple absolute value graphs, however you have to find where the straight lines break. The graphs look like this: