When you take a snap shot with your digital camera your camera also saves some extra meta information to the file called Exif data.
Different camaras save different information to the Exif portion of your picture most of the time it will contains information like Manufacturer, Model, Orientation, Software, Date and Time, YCbCr Positioning, Compression, x-Resolution, y-Resolution, Resolution Unit, Exposure Time, FNumber, ExposureProgram, Exif Version, Date and TimeCompressed Bits per Pixel, Exposure Bias, MaxApertureValue, Metering Mode, Flash, Focal Length, Maker Note, FlashPixVersion, Color Space, PixelXDimension, PixelYDimension, File Source, InteroperabilityIndex, InteroperabilityVersion, ect.
Flickr keeps a database of all this data including manufacture and model and creates trend data.
With this information you can see the rise and fall of different camaras based on there models. Invaluable when you are doing research for a new camera. Flickr even takes its one step father by providing you example pictures taken with the camera.