Beyond distinguishing different types of sources (scholarly, substantive news, etc.), you'll often find that there are multiple types of documents within the same publication. Often the databases will note what type of document an article is, and many databases will also allow you to limit your search to exclude certain types of documents. Here's a brief overview of the types of articles you might see in your search results.
Article - The most common type of document in a publication. Articles generally report news, information and facts, and usually do not include the authors' opinion.
Book Review - A review or critique of a recently published book. Scholarly journals often contain reviews of books published in that discipline, although many newspapers and magazines also publish book reviews.
Brief Article - A short news article, usually no more than 300-400 words
Commentary - An opinion piece written by an individual. Some publications have commentary writers on staff, and many publications allow guest writers (not employed by the publication) to submit commentaries.
Editorial - An opinion piece that may be written by an individual or the publication's editorial board.
Interview - A conversation between a writer from a publication and an individual. These may be published in their entirety, or cut down for space or editorial considerations.
Letter - Usually submitted by members of the public who want to articulate their opinion on a specific issue or issues.
Opinion - A piece containing the author's opinion on an issue. (Many other types of documents - editorials, commentaries, etc. -- could fall under the "opinion" umbrella.)