Here's how Wikipedia defines it:
In political campaigns, an attack ad is an advertisement whose message is meant as a personal attack against another candidate or political party. Attack ads often form part of negative campaigning or smear campaigns, and in large or well-financed campaigns, may be disseminated via mass media.
Televised attack ads rose to prominence in the United States in the 1960s, especially since FCC regulations require over-the-air commercial TV stations with licenses issued by the FCC--effectively all regulated TV stations, since others would either be public television or be pirated--to air political ads by both parties, whether it be attack ads or more traditional political ads. Although cable television and the internet are not required to air such ads, attack ads have become commonplace on both mediums as well.