Art lenses

How are Art lenses different than normal lenses. I see that a lot of the sigma lenses have an 'Art' added to the lens description, how is it different than a non art lens?

Thanks,
Chaks

The art designation from Sigma is their higher quality lenses in their new line. They have designated categories of Art, Contemporary, and Sport. Any Sigma lens with these designations is compatible with the Sigma dock, which allows the owner to perform microfocus adjustments between their camera and lens, and to update the firmware within the lens.

More details here:

http://www.sigmaphoto.​com/lenses/lenses-by-line/