This is one of the most important things in Photoshop. I think it has to be known when using it, and it's usually used in most images.
Brushes are simply 'already made shapes' you can use for absolutely anything. There aren't much more to explain, just try different things and you'll find out everything you need to know.

Before you can do anything you should probably know how to use brushes. Photoshop already has some brushes installed which I will use as an example.
You'll probably find the brushes different places in Photoshop CS2 and CS3. First of all make sure you've marked brushes under windows.

You should then have the Brush Presets popped up, feel free to contact me if you need help here.
My Brush Preset looks like :

Along to the left you'll find different things you can use to change the brush in different ways (brush tip shape - below). You should try out different things here, explore!

Lest go further on, click Brush tip shape.
Here you'll be able to rotate, resize and choose what spacing you want between each brush.

Brush size:
To change the brush size, choose a brush from the sqared selection area.

Now under Diameter there's a draggable area. Try dragging it abit back and forth, or just simply type in the diameter you want on the brush in pixels (px). Do this where it says 59px on the image below.

Rotating brushes

It's quite simple and logic how to rotate the brushes. You'll see this cross with a circle around itm with an arrow in one end. Try dragging this around, you can rotate your brush any way here.

Look at the little dots on the end of one of the lines in the cross..

Try dragging these against the middle. You will then turn the brush like you see it from another perpective. Sorry if you don't understand, but look at the image below and have it a try and you'll get it.

Brush space
This is just basically the space between the brush before it's repeaten, just try it once and you'll get it. Look at the examples below.

I have used Photoshop CS2 here.
Adding brushes is truly really easy. Just follow the steps.

1. Go to the "Palette Dock" looking like the image below.

2. Click the part where it says brushes and you will see a little arrow beside where it says "Brush".

3. Click the little arrow and you will get alot of choices below. Choose Load Brushes and a new window will pop-up.
In this window, find your file with the brushes you want to add, a .abr file.

4. Now click the file, and after that, Load. Or just double-click the file.

The brushes should now be added and ready for use.

OR you can just simply drag the files (.abr files only) into Photoshop and they should be added to the brushes list.

Install from stamps
Stamps are simply images which can be used as brushes, anything can really be used as a brush.
It's very easy to do this. Start by opening the stamp you'd like to use. I will use the one below.

Go Edit >> Define brush present just type a random name or whatever you was as brush name.
The brush should now be added and usable, found with all your other brushes.

Wanna publish brushes for your visitors? You'll need to make brush packages, also known as .abr files.
Start by using Define brush present (under image - also shown above) on all the brushes you want in the .abr file. You need them added to all your brushes in Photoshop before you can make them all one file (the .abr file).

Go Edit >> Present manager.

A new window will appear. Hold in CTRL and click the brushes you want in the .abr file. Make sure you mark more than one brush, that's what makes it possible to have alot of brushes installed when opening the .abr file.

When you've marked all the brushes make sure you click Save set. Just name the set something and save it someplace you'll remember.
Upload it, eat it, sit on it or delete it. Do whatever you want with it, but what would be smartest would be to share it.

Hope it helped !