Well so far it's taken me 120 pages to describe Quicksilver :) It's hard to do in a sentence but it is possible in two paragraphs. It gives you quick access to things on your mac; not just things you see in the Finder like files and applications but often the next level down such as songs, artists, playlists in iTunes and contacts, email addresses, phone numbers in Address Book, and bookmarks in your browser and albums in iPhoto, etc. And unlike other launchers you can do more than just open things. There are verbs to choose like Email To..., Search For..., Lookup in Dictionary, Move to..., Scale Image..., etc. These combine in a lot of interesting ways and you can bind keystrokes to whole or partial commands (kinda like macros). I often highlight an address in a web page or a document and type a keystroke to have a google map appear. And it's pretty and there's utility to it. If I type "John" I see a picture to indicate to me that I have the right John and if not I keep typing a little bit until I do.

But it really took a week's worth of using Quicksilver for me to get it. It's become the user interface for virtually everything I want to start (or often do) on my mac. There's something intangible and wonderful about that. Common look and feel takes on a whole new meaning. Command line people understand it, but for many tasks that use another application other than Terminal (look up something on a particular web site, control iTunes, email a file to someone, etc.) I find Quicksilver faster. Partially because it learns what you do and recognizes that faster, and partially because it's easy to add to configuration for keystrokes and mouse gestures to get to frequent commands. It's always there when you need it, just at your fingertips. When I'm on a different machine I'm very frustrated at how slow doing things the "normal" way is. This is why people say you just have to use it to understand.

My favorite demo of QS's power is this screencast of taking a screenshot, scaling it and posting it to Flickr. This is one of the more extreme uses but it gets the point across.