When opened, it will create a catalog of applications and some frequently used folders and documents.
Activate it, and you can search for and open anything in its catalog instantly. The search is adaptive, so Quicksilver will recognize which items you are searching for based on previous experience. It also supports abbreviations, so you can type entire words, or just fragments of each. When not in use, Quicksilver vanishes, waiting for the next time you summon it.
Mac OS X 10.6 or higher. See home page for older OS X compatible versions.