A framework for drivers of video cameras, TV tuners etc. The device API for userland processes is commonly called v4l or v4l2 (v4l being deprecated in theory). Also known as V4L.
DeviceDrivers written for this framework are normally compiled into the LinuxKernel as Modules. You must have the videodev Module loaded as well. The devices are enumerated by UDev and appear as /dev/video0, /dev/video1 etc.
Some applications require a specific device name e.g. /dev/video, although this is probably not the desired behaviour and these apps will probably be modified in the future. If you are using an application that wants a /dev/video but your system only has a /dev/video0 then you need to either make a symbolic link or set up UDev appropriately. For the latter, create/modify the file /etc/udev/rules.d/00_video4linux.rules and make sure it contains the following:
# make a symlink to the first video device KERNEL="video0", SYMLINK="video"
There are some useful links here:
12 pages link to Video4Linux:
http://www.linuxtv.org/v4lwiki/index.php/Main_Page