Hashtags are used to tag posts. They enable users to perform better Twitter searches and run twitter chats. Despite denigration by Scoble, hashtag popularity appears to be on the rise. As people add more and more Twitter followers, the “noise” level tends to go up and hashtags are a way to sub-divide the stream. (NOTE: a hashtag filter is better than a generic keyword filter because the extra addition of the ‘#’ character shows intention to classify a post whereas otherwise the keyword could be in a post out of context)
Because receivers are starting to filter by hashtags, Tweeters are starting to use them so that their messages will be read by the target audience. Unfortunately, where there is a target audience, there is going to be SPAM. People who are selling products (sometimes completely unrelated to the hashtag) are now adding a variety of hashtags to their messages.
The next evolutionary step in the process is that viewers will soon have the ability to filter both keywords AND people. You will be able to block out that SPAMMER so they no longer show up in your Filter. You will be able to white-list particular people who you do want to get through your filter. Tweetchat has recently added the ability to feature and block particular usernames from a hashtag filtered chat.
If you are interested in finding a hashtag chat, there is a schedule here.