I have been asked to elaborate on the use of hashtags in Twitter. Using a hashtag allows you to tag your post to a specific topic (eg #spconf) You can use established tags to participate in an event or topic (eg #coralinereview was used when people were tweeting their reviews of Neil Gaiman's movie Coraline), or you can start your own (eg #SC_todo). Remember Twitter only allows for 140 character tweets, and the hashtag count towards that limit!
Now that you have tagged your tweet, what's next? You can go to Twitter's search page (http://search.twitter.com/) and enter the hashtag to see all tweets tagged with that one.
There is a site called Tweetchat (http://tweetchat.com/) that uses hashtags to create an auto-updating chatroom to follow the topic. Also, anything you post in Tweetchat is automatically taged with the topic. Disclaimer: I have not used Tweetchat! There is no real information on the site, and it asks for your Twitter userid and password. This sort of thing makes me twitchy, and I wouldn't use it.