it is used to color in part you select. For a good tutorial on how to use the a tool, click here
The Photoshop Brush tool is a basic painting tool. It works like a traditional drawing tool (pen, pencil, brush) by applying the color using strokes.
Here are some useful tutorials:
- Create Your Own Brushes in Photoshop - Creating your own custom brush is really quite easy. You can use anything from scanned textures, photographs, or drawings to make new brushes.
- How to Install Brushes in Photoshop - The Photoshop brushes are saved with the ABR file extension. To use Photoshop brushes you have to install the ABR file in Photoshop. The PS brushes work in other software tools like Gimp.