Twitter (twitter.com) was developed with a single goal in mind, to keep your friends, family, associates, others, etc. up to date with what you are doing. There are 2 main areas to Twitter.

1. What you are doing.
2. What others that you are following are doing.

That's it! It is that simple. Because of its simplicity, people and companies are utilizing it for different use cases. Here's a couple of scenarios for how people/companies are using Twitter to communicate. It's very common for people to have multiple Twitter accounts - some for personal use and some for business use. Who they allow to follow depends on who they want to see their updates.

Personal Communication:

• John wants to go out to eat with friends. So he adds "I want to get a bite to eat at 7pm tonight. Somewhere in Addison. Anyone interested?" Then his friends who follow his Twitter updates will see his update and thus the conversation starts.
• Mary just got a new job with Anderson Consulting and wants all her friends and family to know. So she adds "I got a new job!" to her Twitter account.

Business Communication:

• Microsoft makes frequent updates to their products and so they offer a Twitter account where their customers and others can easily receive notifications when updates are available. So they add "Outlook 2007 SP2 Update has just been released. Go to www.microsoft.com to download the latest update" to their Twitter account. Now all the millions of customers who have signed up to follow that Twitter account are instantly notified of the update.
• The L Group frequently does Executive Briefings that are FREE for those who want to come. So the day after a new Briefing has been scheduled, they add "Join us at our next Executive Briefing on 3/20/2009 at the Hotel Intercontinental. Sign up at www.theLgroup.com."

As you can see, Twitter was designed for SHORT real-time communication.  Messages are usually short in length and, for businesses can be action oriented. How you use it is dependent upon what information and to whom you want to communicate. The possibilities are really endless, because it was designed to be a general communication medium.