From where the vessel is going, where it's been, what sort of cargo it may be carrying, it's nationality, it's current track and speed, it's tonnage and dimensions! All of this and a photo of the boat, lovely.
Upon opening the link, on the left hand side of the page will be a section called Last Position Received. Here click on Current Vessels Track and it will show you on a map just where the boat is and where it's been. Great stuff!!
AIS is a legal requirement for any seagoing passanger vessel, or any comercial vessel in excess of 300 tonnes. It is an amazing system that is aiding in collision avoidance, obviously there is still no excuse not to maintain a visual look out at all times.
The American Coastguard are planning on replacing the RACON system in navigational aids with AIS.