Twitter as it is described on their website: "Twitter is a real-time information network powered by people all around the world that lets you share and discover what’s happening now." If you want to know more about Twitter check out http://twitter.com/about.
Twitter like any social network used for a lot of things, many of which are not really useful for everyone. You would be amazed however how many cool conversations on SQL Server take place on Twitter. Not only is it a super-fast way to get help (use the #sqlhelp tag) but it is a great source of information on anything that is SQL Server related (if you follow the correct people of course). Several people from Microsoft such as myself but also really meaningful people like @powerpivotpro, @dennylee, @buckwoody, @bobwardms and many more are active on Twitter ;-) Since MVPs are real community builders you can imagine that a lot of them are also present on Twitter like @PaulRandal, @KimberlyLTripp, @mrdenny, @frederikvdp, ...
If you do decide to start using Twitter you can find me at http://www.twitter.com/WesleyBackelant. Seesmic has some interesting clients at http://seesmic.com/ like Seesmic for Windows & Seesmic Look.A quick start to find people that are interested in SQL Server can be found on WeFollow at http://wefollow.com/twitter/sqlserver.
See you soon!