This page from the Quicksilver wiki describes the program and its power much better than I ever have.
Quicksilver’s greatest strength, however, is not search. Any item you are able to find, drag, or otherwise pull into its universe is endowed with many potential uses. Hitting
<tab> takes you to the action field, where you can use the same adaptive search to select what you would like to do. Among other things, files can be emailed, copied, compressed. Text can be modified, transmitted between programs, or searched for on the web. Some actions even support an indirect object, so you can send an item to a person, move files to another folder, or open files with a specific application….
In the end, Quicksilver has one very important effect. The effort associated with frequent tasks fades into the background and you are able to act without thinking. After an adaptation period, Quicksilver becomes an extension of yourself; the process fades away leaving only the results.
I realize that this may sound like a pretty fruity, hippiefied way to talk about a piece of software, but it’s actually quite accurate in my experience. Quicksilver may not click the first hour, day, or even week that you use it. But if you spend a little time to learn its subtle tricks, the return on investment is undeniably worth the effort.