The A-Z Dictionary of Educational Twitter Hashtags:
First, What’s A Hashtag?
The # symbol, called a hashtag, is used to mark keyword or topic in a Tweet. Any Twitter user can categorize or follow topics with hashtags.Those hashtags (usually) mean something and are a great way to get a tweet to appear in search results or discussion monitoring.
For example, the popular #edchat hashtag is used by thousands of users every Tuesday. It makes it easy (sort of) for people to monitor what’s happening in the conversation rather than having to try and guess what topics you should search for. By having a conversation on Twitter using hashtags, you also make it easy for any other Twitter user to join in.
Via Gust MEES