The essence of populism is that a large number of people come to believe that an entrenched, venal, and corrupt establishment is governing for its own benefit, rather than for the citizenry at large. Populists seek to overthrow that establishment through elections. The newly-elected government then, in theory, remakes the bureaucracy and other supposedly independent institutions (most notably, the judiciary and the media) to be more responsive to the public will.
Populists can come from both the left and the right. They distrust experts and believe that most complex problems have simple solutions. They put their faith in strong personalities, intuition, and will rather than systems and abstract thought.
Populism is an attitude, not a specific ideological program; populists can be either mostly right or wrong, depending on the circumstances. I will be putting populism in its contemporary and historical contexts over the next week.