I would define populism as a political movement that believes that what the "regular people" want is best. It is based on the idea that the "typical" American's values are superior to those of the elites (and to those of the poor).
Specifically, in the modern US, I would say that populists are relatively (but by no means completely) liberal economically and conservative socially. They tend to want to stick with traditional, middle-class values. This is why they tend to be pro-choice and to be suspicious of gay marriage, for example. Economically, they are not as laissez-faire as true conservatives. They do not like taxes and spending (so they're not truly liberal) but they do want the government to "meddle" in the economy to some extent. For example, true conservatives believe in free trade whereas populists are more likely to believe in protectionism.
So, overall, I would say that populists are people who believe in "common sense" and "traditional" things. They are suspicious of things that highly educated elites tell them because they believe in the wisdom of the "common people."