An example of ad hominem fallacy would be to claim someone cannot argue the benefits of vegetarianism because he is currently enjoying a steak. An ad hominem attack is one that focuses on attacking the individual rather than addressing the actual argument.
Those who employ ad hominem attacks may do so consciously or unconsciously. Those who use using ad hominem attacks consciously attempt to deceive their listeners and turn them against somebody. Those using ad hominem attacks unconsciously may find themselves angrily attacking the personal characteristics of an individual, not necessarily realizing the rhetorical value that the use of ad hominem provides them.