Where books can be broad in their coverage, articles can be very focused. For example: You can find a book on the entire History of the Holocaust.. An article might just be about Camp conditions in Auschwitz.
Google Scholar is another great source for articles (and some ebooks)
*Be sure to go into Settings, Library links and add in Georgetown University the first time you use Google Scholar.
Not all searches are equal: Auschwitz AND "camp conditions" VS Auschwitz AND camp conditions
Content available on a database can include journal articles, book reviews, pamphlets, etc.
Be aware of the concept of peer-reviewed journal articles. These are journal articles that have gone through a rigorous review process by scholars in their field of study. You want to avoid articles that are not peer-reviewed, like popular magazines and many online sources (e.g. blogs, message boards, personal websites, etc.)