It allows visitors to upload historical photographs of an area and tag them with a location and year. These are then displayed on a Google Map.
From the Explore Photos page you can move around the world to find places you’re interested in and then view photographs of what they looked like many decades ago. You can even overlay the old photograph onto Street View to compare today’s scene with the past.
The plan is that if enough people upload enough photographs in enough places, it will weave together a photographic history of the world.
The coverage of the UK is a little thin at the moment, but like all these sites it is dependent on content being uploaded by its users. I’m sure it’ll grow into a very useful resource.
As an alternative – don’t forget to take a look at History Pin.