Tweetdeck is an application that runs via Adobe’s AIR platform (meaning you have to install AIR first).
Tweetdeck is one of several applications that uses twitter’s API, or Application Program Interface, to plug in to the data and backend of twitter, and then use it for their own application. This application could be a standalone program like Tweetdeck, or it could be a service—something you can access from your web browser.
Tweetdeck lets you see lots of twitter feeds at the same time: your main feed, public and private messages, or searches for terms or streams of hashtags.
[...] Then there’s using Tweetdeck [...]