In this dialog you choose which brush you want to use and the settings you want to apply to the brush. The dialog displays all the available brushes. You may also notice some of GIMP's special features like colored brushes (pixmap brushes), brushes with a red triangle in the right corner (so-called "Image Hose" brushes), brushes with artistic text, little flourishes and doodles, etc.
All of those brushes are relatively easy to create in GIMP and save in the appropriate format. GIMP also provides you with a Brush Editor dialog where you can create and edit brushes. 
You will see that some of the brushes in the dialog have a little + sign. The + sign is indicating that the brush preview is scaled to fit the space. To view the real size of the brush, click and hold on the brush square and the real size brush will appear (and disappear when you release the mouse).
When you choose a brush, the name and the size of the brush will appear in the brush dialog. The brush size is measured in pixels.
Spacing is the distance between your brush marks. If you set the spacing to 100, the brushmarks will be like a string of pearls. If you set the spacing to zero, the brushmarks will create a solid, brush-shaped line. If the spacing is higher than 100, the brushmarks will form a dotted line.
Pixmap brushes are small images that can be used as a brush. Since the brush is an image, you can't alter its color. The color setting in the toolbox will not apply to this image.
If you use color as a pressure sensitivity when you paint you will be able to change the color of the pixmap brush.
Default Keyboard Shortcut: Shift-Ctrl-B
you can only edit brushes that are created in the brush editor