A small python script running on a nearby computer downloads messages from the people it is following on Twitter, and sends them to the serial port, causing them to be printed on the paper. It also looks for messages which mention @tweletype, and prints those too.
Anything typed on the keyboard (followed by Enter) will be sent as a tweet from the @tweletype account.
There are actually two pieces of hardware connected to the Tweletype account right now. The first is a TI-745 data terminal, from 1979, which prints on thermal paper. This is still available today as fax paper.
Here's a closeup:
And some Twitter messages:
And here's a zoomed-out picture where you can see the long paper full of tweets disappearing into the background:
The second is a Model 28 Teletype, from circa 1964. This particular unit was formerly the property of NASA and still has their asset tag sticker on it.
It's a bit more imposing that the previous version, being about 4 feet high and made out of sheet metal. It's mostly electromechanical, containing very few electronic parts. The binary encoding and decoding is performed by levers and cams, all powered by a single AC motor.
If you're interested, you can download the source code for the tweletype project at Google Code.
Tweletype: Ce texte ne semble pas exister en Francais, alors nous l'avons remplacé par la même page en Anglais