Mechanical Pencil

A mechanical pencil is a pencil that uses a thin (usually 0.5-0.7mm) stick of graphite coming out of a plastic, pen-shaped holder. It has a button you can press, sometimes on the back with the eraser, that extends the graphite further out whenever you need more.

The benefit is that you always have a sharp tip to write with, and increasing the graphite exposed is as easy as pushing a button. The down side is that if you want a wider mark, you'll have a harder time making it because you can't very easily turn the tip sideways.

Here's a random image off the internets: