Arduino is a framework around a small electronic card containing a weak processor ("as powerful as the one in your washing machine"), a small memory, a USB port, and lots of input/output ports. It was originally meant for artists who wanted to have electronic devices in their creation, but had no notion of electronics. Basically, you plug several of the existing extensions (LED, movement detector, ...), write a small program in a simple C like language, upload it to the card and start it. From then on, your imagination is the limit.
One of the exemples he showed us, and this is where it linked us back to Java, was a Minitel (a kind of very limited computer, the french version of the Internet in the 80's) linked to a computer through the card, where a Java server was running listening to the serial port, that allowed you to tweet..
Since all the parts are very cheap, Arduino looks like a great hobby.