The pixel pencil is a hard-edge square brush. The default shortcut for this tool is N.
There are some basic brush settings that you can change in your panel under the color wheel.
Clicking the gear icon, you have Simple, Advanced, and Full settings modes.
The different settings and characteristics include:
Size
Density
Opacity
Stabilizer
Color
And color jitter.
These are explained in more detail on our Brush page.