Recipe:
1. A makes Claim B
2. There is something objectionable about A
3. Therefore, claim B is false

Why is it a fallacy? Because it uses tactics such as:

1. Abusive name calling
2. Circumstantial guilt by association (Oh thats because you are a ____)
3. Two wrongs make a right (Quoquo)

Examples:
Lawyers- Using financial backgrounds. i.e. Oh he stole because he was poor
Gender- Sexism, its only a mans job, or. You are only a woman.
Age- You're too young to understand

Economic Example:
"President Clinton has great economic policies; just look at how well the economy is doing while he's in office" This addresses him and how he conducts himself as a president rather than his policies

Sources:
S. Morris Engel, With Good Reason: An Introduction to Informal Fallacies (Fifth Edition) (St. Martin's, 1994), pp. 198-206.
Shermer M. The Science of Good & Evil: Why People Cheat, Gossip, Care, Share, and Follow the Golden Rule. New York: Times Books; 2004.
"Ad Hominem Examples." YourDictionary, n.d. Web. 19 August 2015. .