The Brush Tool comes with plenty of options. In fact, it has so many options it gets its own dropdown dialogue box and panel where you'll find all the controls split into sub panels. One of the most useful options available to any Photoshop user (not including the resizing controls) is brush hardness. Here I've prepared a few illustrations and a little bit of text to show you how it works.
What does brush hardness mean?
The first thing to know is that a brush is either hard or soft, and the actual hardness is measured in percent. As you can see in the first illustration below, a brush with 100% hardness has a solid edge similar to the effect you'd get if you took a real-world paint brush and applied a stroke to a wall. While a brush with a 0% hardness has a softer, feathered edge more reminiscent of a spray can.
How do I change the hardness of my brush?
Brush hardness can be adjusted either from the dialogue box, the brush panel, or the keyboard. Here's a few ways, starting with what I believe to be the easiest-
1. I'm a big fan of keyboard shortcuts, especially when they save time and effort. To change brush hardness from the keyboard, simply hold down the SHIFT key and press either the RIGHT BRACKET KEY to increase the hardness or the LEFT BRACKET KEY to increase the softness. This works in 25% increments, so if you start with a brush that's 100% hard, and press the SHIFT + LEFT BRACKET KEY 4 times, you'll end up with a 0% hard brush - better known as a soft brush!
2. With the Brush Tool active, RIGHT CLICK (or OPTION click on the Mac) any area of the canvas to display the brush size and hardness settings. You can change either the brush diameter or hardness using the sliders. This way also gives you more control as you can change to any percentage and are not restricted to 25% increments like in the previous suggestion.3. Available in Photoshop CS4 only, SHIFT + ALT + RIGHT CLICK (COMMAND + CONTROL + OPTION + LEFT CLICK on the Mac) and then drag left or right to adjust the hardness and see a live preview of what changes you're making to your brush.
4. When the brush is active you'll see some of the brush options available in the options bar. To the right of the word 'brush' you'll see a little arrow pointing downwards, click it and you'll get options to resize the diameter and hardness as you did in suggestion 2.
5. Go to the Windows menu and open up the Brushes Panel. Click on the 'brush tip shape' option and you'll see the hardness slider. This only really comes in handy when setting up a complex brush with some of the other options the Brushes Panel offers.
When and why would I change brush hardness?
Brush hardness works with all tools that utilize the standard Brush Tool, such as healing, cloning, burning, dodging and sponging. Many times you'll find it helpful to add softer transitions to a stroke, such as when toning skin with the Burn Tool, or erasing the background elements of a photograph.