Tweetchats are interactive community events using twitter. The idea is that participants follow one given hash tag, which has been communicated upfront, e.g. in a blog. Example is #SAPPortalchat. Tweets can be followed on any twitter clients, but there are some specialized options too such as http://tweetchat.com. Usually some experts are invited who provide knowledgeable answers and keep the quiality high.
The challenge is to formulate the message in 140 characters. When SAP organizes such a tweetchat, anybody can join and it offers a unique opportunity to virtually meet product owners, developers, product managers, SAP mentors, etc., so the best sources of completely up-to-date information. It usually takes an hour and it gives great insight into current and upcoming technologies.