A database is a collection of sources, often on just one subject, that includes a search engine and other tools.
Most library databases are by subscription, meaning they contain sources not found for free on the web.
To get recommendations of the best ones for you, try our Research Guides in Academic Subjects.
All databases look different, and you should get to know the best ones in your subject & the special tools in them.
EBSCO Databases
ProQuest Databases
Other Great Databases