Why Use Them?
While the panoramic or "wide gaze" approach to research can be good ways to help generate an interest or area of exploration, research projects often require you to look close up at a body of research produced by scholars in a particular field.
This research is typically collected, codified, and made 
findable in a tool called a subject database.
Every academic discipline has at least one subject database that's considered the disciplinary gold standard -- a reliable, (relatively) comprehensive, and accurate record of the books that scholars are publishing, and the ideas they're debating and discussing in important and influential journals.
Databases are like lenses: they change what you see and how you see it -- and they offer you easy and efficient ways to bring your questions into sharper focus.
Good Subject Databases For ...
HISTORICAL PERSPECTIVES
America: History and Life: the premier database for historical scholarship on North American (what is now the United States and Canada), from prehistory to the present.
Historical Abstracts: the premier database in world history, excluding North America (U.S. and Canada). Scholarship covers the periods from 1450 forward.
FILM
Screen Studies Collection: A comprehensive survey of current publications related to film scholarship, theory, and criticism. This collection includes the specialist index FIAF International Index to Film Periodicals Database, a premier resource for film study globally.
LITERATURE and POPULAR CULTURE
Project Muse: A trusted provider of authoritative humanities and social science books and journals from more than 200 of the world’s most distinguished university presses and scholarly societies.
MLA International Bibliography (EBSCOhost): The most important academic database for deep searching of the scholarship produced about all periods of literature (and in all languages). It also has strong and substantial coverage of scholarship on film, popular culture, folklore,and film. If a Literature Department teaches it, you'll find it covered here.
PHILOSOPHICAL PERSPECTIVES
Philpapers (ProQuest)
POLITICAL PERSPECTIVES (including political theory)
Politics Collection (ProQuest)
SOCIOLOGICAL PERSPECTIVES (including social movement theory)
Sociology Collection (ProQuest)