Ad hominem is the name of a logical fallacy that makes a personal attack on the character, background, etc. of a person, rather than dealing with the issue at hand. These personal attacks, true or not, carry no weight with the argument itself. Use of ad hominem can backfire on the abuser because he/she aften appears to lack his/her own strong stance to support. This can have a serious negative effect on the user's credibility (ethos). This fallacy is also called "Poisoning the Well." The idea is that, if a well gets poisoned, any water that comes out of it could be deemed undrinkable. Likewise, if a person's character is "poisoned," the audience might also conclude that all of that person's ideas are illogical. While this metaphor doesn't work perfectly (see the faulty/false analogy fallacy), the connection is understandable.
Two subcategories:
This version of the ad hominem fallacy is pretty self-explanatory. When a person is attacked with abusive namecalling, it is his/her ethos that is being targeted, not the logic of his/her premise. Using this type of fallacy can make the attacker look immature, unfocused, and desperate to the educated audience. That said, in an emotionally charged argument, it can further charge those who already support the abuser and, in some instances, can sway the uneducated portion of the audience.
Example: "Your Honor, it is important that you find Mr. Spagnolo guilty of speeding! He smells funny, isn't handsome, and once yelled at a homeless man."
Example: "You don't like CANDIDATE X because of her policy on immigration? Your an idiot."
To be clear, I spelled "you're" incorrectly on purpose. :)
Circumstantial ad hominems occur when someone's argument is dismissed because he/she appears to have a conflict of interest. While it is true that he/she may have a conflict of interest, that doesn't necessarily mean that his/her argument is without merit.
Example: Your argument that the United States is the best country in the world is false because you're from the U.S. and have to say that.
Example: Of course Mark Zuckerberg believes governments should pay to make sure that every person has an internet connection; after all, he owns Facebook and stands to make a lot of money if everyone can access the Internet.
Yes, Mark Zuckerberg would make more money if more people had Internet connections, but does this mean that his argument is inherently bad? Not necessarily. It should be taken on its own merits regardless of whether Zuckerberg stands to make money from it or not. Frankly, because Zuckerberg really understands the uses of the Internet, he might actually have some ideas worth listening to. In theory, it can be an honestly good idea AND it can make him money. Those results aren't necessarily at odds with each other.