Frontend, n. the parts of a client/server application executing in the browser.

Backend, n. not what it sounds like. The parts of a client/server application executing on the server.

Database, n. Strictly speaking, a database is a structured repository of data. Broadly speaking it is the persistent storage of the data of your application.

Full stack, n. all of the above. A full stack developer masters the frontend as well as the backend including database technology.