A database is a collection of objects such as tables, views, and stored procedures. Although a typical SQL Server installation has many databases, it always includes the following three: master, model, and tempdb. (An installation usually also includes msdb, but that database can be removed.) Every database has its own transaction log; integrity constraints among objects keep a database logically consistent.
Databases are stored in operating system files in a one-to-many relationship. Each database has at least one file for data and one file for the transaction log. You can easily increase and decrease the size of databases and their files either manually or automatically.