I was amazed to encounter some educators recently who did now know what hashtags are… here is the answer I gave them.
Hashtags are terms added to tweets that follow a # sign. These can be used to indicate the subject of a Tweet, so that different users can contribute to a topic; users can follow hashtags, search hashtags, and embed timelines of all the tweets that were marked by a particular hashtag.
There are no “official” hashtags, they are created by individuals and some become very popular very quickly. Many conference organizers will announce a hashtag and print it on programs and other materials so they can collect and share tweets about the conference. Instructors can even create their own hashtag for their course. By using this hashtag, students who have Twitter accounts will be able to contribute to the timeline as well as faculty. Of course, there is no way to control who uses a hashtag, so your timeline based on a hashtag may display unwanted tweets.