An opinion piece is an article, usually published in a newspaper or magazine, that mainly reflects the author's opinion about a subject (as defined by Wikipedia).
Opinion articles are based in fact and/or information from reliable sources, but display bias and/or support towards one side of an argument. They are a way for journalists to express their personal thoughts on topics or events side-by-side with facts. These articles usually have argumentative and persuasive elements. A lot of them offer new viewpoints and often controversial topics.
Many newspapers, such as The New York Times and The Wall Street Journal contain opinion sections! By separating informative and opinionated styles of writing, they can publish both without confusing readers on which is which.