Go to the library website (if you know the database title) or the A-Z Database list (if you want to browse all databases)
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 Hundred Years War that covers the entire duration of the war. An article might just be about the Battle of Poitiers.
A good databases that has broad coverage of journals in history is JSTOR
Not all searches are equal: "battle of poitiers" AND "black prince" VS. battle of poitiers AND black prince
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 You want to avoid articles that are not peer-reviewed, like popular magazines and many online sources (e.g. blogs, message boards, personal websites, etc.)