I was trying to use photoshop and I noticed when I chose a brush and changed the flow and opacity, these settings were used for all other brushes I selected. So after some trial and error I think the tool preset is a way of saving the brush settings just for that particular brush. This way I could select different brushes from the preset menu and the different brushes would have their own flow and opacity settings.
Here are a couple of doodles I made with brushes that I changed a little the flow, opacity, darken mode and shape dynamics control to pen pressure. I then saved the brush in the tool preset by clicking the box on bottom right.
I enjoyed trying out the different brushes that come with Photoshop and tweaking the ones I liked a little like I have one for rough drawing (with opacity low I can draw and build shapes). I also liked a couple in the natural brushes to use for blocking in a soft color. Here is the tools preset I made for Adobe Photoshop CS3 (1.3MB) (https://dl.dropbox.com/u/91671011/na_pens.tpl) to install it just go to the preset and click the list and load the preset (see pic ).
(for a mac i think the default search for tools preset is something like /Users/noa/Library/Application Support/Adobe/Adobe Photoshop CS3/Presets/Tools ) so i think putting the file here will let Photoshop find it quickly next time around.
Hope this is helpful.
Inspired by Jim Stigall's tutorials on ink brushes (jimstigall dot blogspot dot com)