New to the freelance writing game? Be sure you know the lingo. Here is a nice glossary of writing terms from freelance writer Amy Derby at write-from-home.blogspot.com.
I'll add a few definitions:
Essay - A non-fiction piece of writing that expresses a point or lesson from the author's viewpoint.
Op Ed - A piece of writing expressing the author's opinion on a subject. They are usually written for newspapers, but may also appear in other publications.
Sidebar - Writing that is set off from the rest of the article in the margins. It can be a recipe, a highlight of a quote from the article, a quote from someone else, or additional facts and information on the subject.
The DabblingMum has an article on "How to Write Sidebars". If you pitch an article to a magazine that likes them, be sure include suggestions for the sidebars. The more work you can do for the editor, the more he or she will love you.