I just finally purchased my own wacom digital drawing tablet and started some digital illustration. I'm just beginning to become familiar with the pen and all the various digital brushes. What the wacom pen offers that the convential mouse does not, is pressure and tilt control. When using a mouse, my options for painting strokes are simply on or off. With a tablet, I can control things like a stroke's opacity, size or angle simply by how hard I push or tilt the pen while drawing.
The image above is my first attempt at digital painting in Adobe Photoshop. I started with a basic sketch and some blocks of color to frame the composition, and then began sketching the details in with the pen. The image consumed about an hour of my time, and because of it's speed and stylistic approach, this has become known as "speed painting".
The image on the bottom shows the difference between brush strokes with a mouse vs. a wacom pen and tablet. Each plotch of color was made in a single stroke.