Brushes are the backbone of your graphical program, you can use brushes to draw lines but also use them to change lightning or patterns. Photoshop brushes can also take the place of clipart, enabling you to instantly add just about any object, plant, animal, person, or thing you can imagine.

From Photoshop 7 (the last one before the Creative Suits in 2003) or later you can use brush sets. Most Photoshop alternatives (Corel Photo-Paint, PaintShop Pro, GIMP, etc.) are patterned after Photoshop to some degree, and thus many can use Photoshop .ABR brush sets.

Here you will find a couple of nice tutorials and sources.