Twitter, a micro-blogging tool, was designed to be compatible with text messaging in mobile phones that is why only 140 characters maximum is allowed for each update.
Twitter can also be considered a cross between a chat room and instant messaging.
It is actually an open forum but you are allowed to restrict it to the people which you want to connect with.
By using it more, you will be able to get more followers and the same time you can follow more people.
Listed below are a few ideas for using Twitter for PR and marketing purposes:

• Keep your CEO engaged in social media.
  One good way to communicate with your market is through social media where you can manage connections with customers, bloggers and prospects.
  However, the CEO of a global and large company will find the typical routes of social media hard to follow since he or she only has little time available for writing blogs or answering messages.
  But since Twitter only needs short comments and thoughts, your CEO will find Twitter ideal for sending text messages about his interesting opinions about your market.
  He can actually send messages whether he is meeting with people or on the road.
  
• Communicate with the media or bloggers.
  Following someone on Twitter is very easy.
  And you will often get the surprise of your life when you discover that a lot of people have decided to follow you, too.
  As a matter of fact, Twitter is a much better way to get connected with influential media people rather than sending them emails or making calls.
  
• Announce sales, deals and specials.
  You can expect a lot of success if you make instant announcements to a large audience like that of Twitter.
  This can be considered a good marketing scheme.
  
• Post live updates on conferences and other events.
  Twitter is a good tool for making announcements on last minute changes if you run your own corporate events or participate in large trade show exhibits.
  
• Promote interesting news, webinars, blog articles and a lot more.
  Posting articles such as PR coverage and customer success story on other websites which are relevant to your business can offer a lot of advantages to your business.
  You can also post links to other contents which could attract your audience like a free webinar.
  

Again, these are just a few ways that you can make use of Twitter for marketing purposes.
The list can still go on and on depending on the needs of your business.