Automatic Identification System (AIS) is a collision avoidance system that gives information all the ships in your area, their speed and courses and how to contact them (name, callsign, MMSI). This information is publically broadcast on VHF radio which can be picked up either by other ships or by shore based receivers.
AIS works best over a range of a few miles as the AIS signal is more or less limited to line of sight to the horizon (usually 10-20 miles). However, by getting together a group of amateur shipping enthusiasts around the country equipped with suitable receivers and aerials, ships can be tracked over longer distances.
Where there are many receivers, ships can be tracked continually. However, where there are only a few receivers or it is not practical to place a receiver (eg far out to sea), then there will be gaps in the coverage. UK coverage is not complete or continuous. There are gaps in the coverage and not all areas are covered 24 hours a day.
Looking at this on the website, a ship will initially seem to stop (The Received: line is not updated) and it's colour will be draw in a pale colour. Eventually the ship may be removed from the map. When a new AIS signal is picked up from the ship, it will jump and be redrawn in a new place, but in the full rich colour.
Although I try to be accurate, there are various possible sources of error with AIS. These may be as simple as the crew entering incorrect details into the AIS transmitter, but they can also be caused by radio interference can corrupting the AIS signals. This may cause phantom ships to suddenly appear, for real ships to have incorrect details or for a ship to get another ships details.
Basically, don't trust everything you read on the internet.
My original and professional interest is in image processing and I wanted to experiment with a webcam. Most of my view is of other peoples houses, but there is a narrow gap where I can see part of the sea. I ran a webcam for about a year experimenting with motion detection software and slowly gathering a collection of nameless shipping pictures. I was then introduced to AIS as a way of identifying these ships. As my interest in AIS grew and other people started contributing AIS data, I added more maps and the popularity of my website started to rocket.