Here's what I say:

"First: at a very basic level, it's a launcher. If you want to launch Firefox, all you need to do is type Ctrl+Space, F-I- (Oh, hey, it figured out that I probably want to open Firefox!) ENTER.

"However, Quicksilver is far more powerful than that. In addition to indexing the programs you have, it also indexes lots of other things; for example, your Address Book. If I want to write an email to Mary Smith, I invoke Quicksilver (Ctrl+Space), start typing her name (M-A-R-Y) and her name pops up. I hit TAB to get to the other pane, and type C-O-M-P. Before I've even finished typing "compose", Quicksilver has figured out that I want to compose an email to Mary Smith. I press ENTER, Mail opens and a new message to Mary is brought up, ready to go; all I need do is put in a Subject and write the message. All this, without ever having to leave the keyboard!

"What I'm saying is this: Quicksilver is far more than just a program launcher. It's a tool to let you do things, to get to a task quickly and without having to hunt through the Applications folder or your Address Book or your Bookmarks. It does this with simple typing. It's a very, very smart program and it's only for Mac. Once you get used to it, you'll never want to be without it. When you then go and use a Windows computer, you'll find something severely lacking."

I generally stay away from the whole "sentences" metaphor, because an average person will likely have trouble understanding the concept without a lot more explaining.