Trade or professional sources are written for an audience of professionals in the field. They may resemble scholarly journal articles or popular magazine / newspaper articles. Some library databases will include scholarly, popular, and trade sources, so you'll want to be able to tell the difference between the three.
Explore the image below -- mouseover the targets to learn more about the anatomy of a trade publication. This example uses an article from a trade journal, but the characteristics discussed also apply to some magazines, and newspapers.
Callari, James J. "Making Old Bags New Again." Plastics Technology (2012): 36-41. Academic Search Premier. Web. 30 Jan. 2013.