Ad hominem or against the person is an invalid argument form, and logical fallacy, that discredits a person's claim by attempting to undermine their credibility.
The argument generally looks like this:
Individual X makes a claim, P.
There's something wrong with Individual X (or Individual X is not credible).
Therefore, P is false.
Ad hominem arguments are invalid because the truth of the premises do not guarantee the truth of the conclusion.
Joe believes that Martians have visited Earth.
Joe is insane.
Therefore, Martians have not visited Earth.
In this example, the fact that Joe is insane does not mean that the conclusion follows from the premises. Although Joe is likely not a credible source for information, it is possible for a crazy person to believe something that is true, just as it is possible for a sane person to believe something that is false.
Ad hominem circumstantial arguments claim that someone is likely to take a particular position (or make a particular claim) because of their circumstances. For example:
Of course, those politicians will reject public health care. Their campaigns are paid for by lobbyists!
Ad hominem abusive arguments make claims against a person that are also meant to undermine them, but are usually excessively insulting, and often irrelevant to the claim that they're making. For example:
Sue says that the Earth orbits the Sun. But Sue is stupid and nobody likes her, so she must be wrong.
"You too" arguments are a type of adhominem that attempts to discredit someone's normative (or value-based) claim by establishing their hypocrisy. For example:
You keep telling me that eating meat is wrong, but the other day I saw you with a cheeseburger.