A DDL (Data Definition Language) file contains commands used to define the structure of a database. These commands are part of SQL (Structured Query Language) and are used to create, alter, and delete database objects such as tables, indexes, views, schemas, and users. DDL statements define the metadata of the database, essentially describing the blueprint of the data storage. Common DDL commands include CREATE, ALTER, DROP, TRUNCATE, and RENAME. The specific syntax and features supported may vary depending on the database management system (DBMS) being used, such as MySQL, PostgreSQL, Oracle, or SQL Server. DDL files are crucial for database administrators and developers to manage and maintain the database schema, ensuring data integrity and efficient data access. They are often used in conjunction with Data Manipulation Language (DML) commands, which are used to insert, update, and delete data within the defined database structure.