If you've never used Twitter before, you might wonder why everyone likes it so much. Here's perhaps the most coherent, simplest explanation I've seen so far:
I suppose if you don't "participate" in the online world and are mostly a passive consumer, Twitter might not seem all that interesting to you. However, if you're actively blogging and participating in the conversation around the things that interest you online, Twitter is an indispensable tool to get you closer to those in that community.
Personally, though I also use Jaiku, which is now owned by Google. It has a rich client for Nokia S60 handsets. Because it supports conversational , it's a bit easier to follow more people.