Difference between database and database instance?

In general, a database is a collection of related data. In more specific terms, a database is typically a collection of computer files, usually stored on disk, used to contain and manage this data. The logical structure of a database (as organized in tables, indexes, views, etc.) does not necessarily correspond directly to its physical structure (the data files, transaction log files, etc.).

A database instance is the instantiation of memory structures and processes by a database management system manages the logical and physical structures and the data organized and stored therein. An application program must connect to and interact with the database instance in order to insert, retrieve, modify, and/or delete data stored in the database.