Much like the anti-hero, the anti-villain goes against the typical character traits that would normally embody a villain. Where the anti-hero means to put a darker take on the squeaky clean hero, the anti-villain means to humanize the evil world dominating villain.
Most anti-villains are aware that what they're doing is not necessarily good, though they may be reluctant to dive completely into the "evil" and relinquish the part of themselves that still hold some sort of moral fiber. Or perhaps it could just be a small hint of cowardice to completely become the evil that they thought they could handle.
Anti-villains could also have large gray areas in their moral code. In all other aspects, they would be considered the villains because they exhibit qualities normally seen in villainous characters (i.e. stealing, cheating, killing, etc.). But the gray areas are what sets them apart from a true villain. These gray areas may be fueled by love for friends and family, and/or loyalty to the same.
For a more in-depth explanation of the anti-villain, mosey on over here.
How do you feel about anti-villains? Do you think they make more effective villains than true villains?