A Twitter chat is a an organized event (sometimes occurring at regular times) to tweet on a pre-arranged hashtag. It is similar to a Twitter party, but while a party may only occur once or twice, Twitter chats frequently recur at the same time weekly or monthly.
For example, #gtchat is the hashtag for a weekly Gifted and Talented Education chat that is held every Friday at 4pm PST.
You can view and/or join the chats using any Twitter client that supports a hashtag search, including the Twitter.com website itself.
Here are a few more Twitter chat resources.
Twebevent is a website that lists and promotes Twitter chats, and also provides an easy tool to view past chats, as well as upcoming events.
The Twitter Chat Schedule. Robert Swanwick hosts a Google spreadsheet listing more than 750 Twitter chats. If you know (or host) one that isn’t on his list yet, feel free to add it. The public Google doc is editable by anyone.
For even more hashtag and Twitter chat tools, read this list from Kevin Mullett, for 35 more tools to help you get the most from Twitter chats.