A database is a centralized collection of data placed in one or more files. Multiple application programs can access this data concurrently; consequently, redundant files are not required for each application. Application programs running in batch, time-sharing, and remote job entry environments can all access the database concurrently. A database consists of the following major components:

A data set, set, or subset that is not an item of another data set is termed disjoint. Structures need not be disjoint; that is, a hierarchy can exist between the various data sets, sets, and subsets. A data set, set, or subset that is an item in another data set is said to be embedded. When a database contains embedded structures a hierarchical file structure results.