Twitter is a free online service where each user has his or her own webpage that they update as often as they want 140 characters or less at a time. Users update their pages with news, conversation, resources, questions, quotes, humor, and sometimes useless information. People often call these short posts updates or tweets. Many educators use Twitter to connect, collaborate, ask questions, and build a sense of community. This is called microblogging because it's like having a blog where the posts are very short.
Twitter HashTags Explained http://TwitTalk.tv with Mike Wesely explaining what is the Hashtag on Twitter.