A populist politician is one who is devoted to, either sincerely or insincerely, "telling the people what they want to hear". Critics of populism, or those who favor elitism argue that the best ideas are not likely to be the most popular.
Populism can incite class warfare or xenophobia where the politician whips up anger against a minority based on wealth or ethnicity. See: tyrrany of the majority.
Alternatively, at its best populism serves the public interest by nurturing ideas which run contrary to the vested interests of the wealthy and powerful, such as reducing income inequality.