Here's what the interface looks like (image copied from their home page):
The interface is designed to show your multiple streams at once. What this means for you is that you can review your standard stream, per search term, and direction messages to you, any mentions of you, any suggested Twitter's to follow, and so forth. You can setup a new column to search particular topics using search term, including the use of hash (#) subject delimiters. Your search terms are then saved so that when you close a column and wish to return to it in the future, you don't have to type it in again. You just bring up the term under the Quick-Add Saved Search terms of your Search screen (See below).
What I like most about TweetDeck is the ability to view and publish posts to Facebook and MySpace simultaneously or selectively. In other words, I can send the same message to one or more of them all at once! That's cool.
What Do You Think?
If you use TweetDeck and you'd like to comment on its usability, or if you prefer another tool, feel free to comment or vote here.