Hashtags are just words added to tweets that act like topic or event labels. They let you search more easily for tweets by topic. Even before you start tweeting yourself, hashtags are a fun and useful way to get value out of Twitter. To turn a word into a hashtag topic label, all you do is add a “#” to the beginning of it (so iPhone would become #iPhone, for example).
One of the coolest uses of #hashtags is to create Twitter Chats. To talk about the topic, just add the topic hashtag to your tweet (such as #BlogChat or #ToolsChat). They are one of the very best ways get started tweeting. You can just listen, or chime in anytime!
Chats let you participate in discussing a particular topic with other Twitter users, and last only an hour or so. There are Twitter chats for web design, blogging tips, Social Media and much, much more. Here is a great list of hundreds of Twitter chats.
TweetChat makes chats easy for you. Set the dashboard to the chat hashtag you want to follow, and it will automatically show you all new chat Tweets as they are sent.
Cool Feature: The interface lets you hide any user that might be over-tweeting or off-topic to keep things focused and useful for you.