Here is a link that explains pretty well, what it does >
click.
As I've said, I mostly use it as an application launcher. Here's how that works:
You press the hot key you've assigned and the Quicksilver search window opens. Now that it's open, you just type the first few letters of the application you would like to open and once you see the icon of the application, you just hit enter. The application opens and the Quicksilver window disappears.
The great thing is, that it remembers, which applications you use more, so for example, when I want to open Transmission, I just type "tra" and it shows me the icon of, let's say, Transmit. I just go ahead and type "nsmiss" and it will then show me Transmission's icon. After that it's enter, and it's open. The program will remember that, and the next time I type "tra" it will show Transmission's icon right away.
I bit confusing maybe, but you really have to try it to understand. It's simply amazing and amazingly simple and just saves a lot of time : )