A database is an organized collection of data maintained for queries and updates. A database management system manages that collection, enforcing access rules and database integrity constraints. A relational database represents it through database relations, rather than exposing only an arbitrary collection of bytes in files.
For example, a laboratory can store biological sample identifiers, measurement values and scientific instrument identifiers as database tuples. A database query can select measurements above a threshold and join them to scientific instrument records. A foreign key can require every recorded scientific instrument identifier to name an existing scientific instrument. This makes the connection a maintained rule rather than a convention that every application must remember.
The logical relational schema and actual storage are distinct. A database query plan may use an index or scan database pages while returning the same specified result. Database transactions coordinate related updates, and database integrity constraints restrict the permitted states. PostgreSQL's documentation gives a concrete implementation in which named tables contain rows with named, typed columns and databases group tables.[ref]
New to topics? Read the docs here!