A push poll will be introduced like a poll on the phone, but it will be short and contain only negative messages about the opponent and/or positive messages about the candidate. It's really telemarketing. It should not be confused with real polls that include negative information about a candidate. Legitimate political polls must realistically simulate the campaign environment, and so will often include negative statements about their own candidate and/or the opponent in order to test the effect of that message on the electorate. Given the cost of a good poll and the limited number of people contacted (15 minutes x 500 respondents=$15,000) it's too expensive to expect a legitimate poll to have much effect on the electorate.
This is the statement on push polling of the American Association of Public Opinion Researchers: "A push poll is a form of negative campaigning disguised as a political poll that is designed to influence voters' opinions, not measure them. Legitimate polls accurately describe the candidates and issues in order to understand voter reactions. Push polls frequently present distorted information in order to influence voters. Push polls go beyond the ethical boundaries of political polling and bombard voters with problematic statements about candidates or issues in an effort to manufacture negative voter attitudes." AAPOR Statement on "Push Polls." May 19, 2004. http://www.aapor.org