Hard brush vs. Soft brush

I am new at CS3 and I read and hear others talking about using either a Hard brush or a Soft brush. I don't understand what the difference is and how you change from one to the other??

A soft brush is one with edges that fade off, and a hard brush is one that has, well, hard edges. You want to use a soft brush, generally, when editing photos, as a hard-edged effect is going to be obviously faked.

I haven't used Photoshop since CS2, but unless they've changed it, Shift+[will soften your brush, and Shift+] will harden it.