The standard allows a lighting signal to be sent over ethernet topology (using hubs, fibre cables and star-wired Cat 5 installations) and has been adopted by many of the major lighting manufacturers including Zero88, ADB, and Robe.
Artnet enables the sending of multiple (theoretically as many as 16, although 4-8 is a more realistic limit) universes of DMX down a single set of ethernet cables. This data can then be sent directly to the fixtures and dimmers, if they support it, or turned back into standard DMX using an interface box.
The majority of network-enabled lighting consoles have some support for Art-Net, as implementation does not require royalty payments to Artistic Licence.
It is owned and maintained by Artistic Licence Ltd.
Art-Net II was recently published by Artistic Licence and is intended to overcome some of the limitations of the original ArtNet, but is not widely supported.
Streaming ACN (ANSI BSR E1.31) is a considerably more flexible protocol which is expected to replace ArtNet in new installations, with ArtNet becoming a useful legacy protocol.
The Art-Net II specification can be found here (PDF file).