Twitter is the Internet’s hottets mode of communication these days. Like all good ideas, this is crushingly simple. Type in a message of less than 140 characters. You have a group of people (your followers) who’s list of messages this is added to. Your list of messages is what everyone you follow has added in the last few hours. You leave this open in your browser (or, increasingly, on your smart phone) and read in real time what others are thinking or doing.
Originally people posted what they were doing at that moment, and you got a sense of what your acquaintances were all experiencing. More and more, people are posting interesting things they see, hear or find. It is an amazing resource when you need an answer about something you know nothing in a very short time. Ask a question, your twitter followers answer in minutes or seconds.
In class? Set up a titter account for everyone, and have your students follow you. A great way to get feedback during class (put your feed from twitter up on the class projector, your students can comment in real time on your class). If you want a more restricted audience, try Yammer (restricted by company address, like @swu.ac.jp, I have that account), Jaiku, Plurk (with an interesting graphical interface), FriendFeed (twitter in steroids, collects stuff from all your social network sites, the upcoming twitter killer).