Twitter is a free online service that lets you update people on what you're doing with a short message. You can also follow what others are up to.
 
Twitter messages are text-based posts of up to 140 characters, displayed on the author's profile page and delivered to other users ('followers') who have subscribed to them. Messages can be sent and received via the Twitter website (free) or texted via mobile phones. Senders can restrict delivery to their circle of friends or allow open access.