A "Tweet" is a short (<140 characters) message which is added to a list. Subscribers to this list can then see the messages as they come in. It's original use as a Social Networking medium has diminished, but its use has increased on the event & business front. It is ideal when lots of people are commenting on the same event. I saw it used impressively at an Open University conference where the delegates were 'tweeting' reactions to the panel's comments, and the tweet stream was shown on a screen on the platform! (Top Government and BBC staff were on the platform and remarked how useful it was to have a stream of comments rather than just questions at the end.)
Twitter is free and is available on all computers and mobile phones (except, perhaps, the very early models). A person sets up the list by giving it a name eg "housemanhundred". To send a message to this list you would do something like ... "@housmanhundred - Off to recce the last thirty miles this weekend". The "@" sign means "send to this list". To view the list, or "stream", you search for "housemanhundred" within a Twitter program.
As with all such media, it is its collective use that makes it useful - not as prescribed by the organisers. "Great food at checkpoint at CP3", "Fallen tree has crushed bridge at 123456", "Knackered, fed-up, someone tell us a joke", "Plenty of 100'ers in the Inn at Bridge - join us!".
Such twittery is, of course, for those who like this sort of thing! (Garfield)