A Twitter hashtag is a keyword preceded by the # symbol used in a tweet. Hashtags help Twitter users quickly find content related to a specific subject. For example, a Twitter chat about the Olympics might use the hashtag #olympics. Each tweet related to that topic would begin with #olympics.
Using a hashtag makes it easy for anyone looking for tweets related to that topic to find them through the Twitter search funtion. Twitter users often pre-arrange "tweet chats" around a specific Twitter hashtag. Tweet chats can be conducted through the Twitter search feature or through tools such as TweetChat, TweetGrid, or TweetDeck.