A file that is one part of the “data” portion of a SQL Server database. Stores all data such as tables, rows, stored procedures, and indexes. While each database has one or more data files, each data file can belong to only one database.
A collection of tables and other objects. Each database contains a data portion and a log portion. The data portion contains all tables, rows, indexes and so on, and resides on at least one data file. The log portion tracks updates to the data, and aids recovery. Logs reside on one or more log files.