Data Definition Language (DDL) is a computer language used to create and modify the structure of database objects in a database. It is a subset of SQL and is used to define the database schema, which deals with descriptions of the database schema and is used to create and modify the structure of database objects in the database. DDL statements are used to create and modify database objects such as tables, indices, and users.

DDL statements can be used to define columns and data types, and they can also be used to describe, comment on, and place labels on database objects. DDL is sometimes known as Data Description Language since its statements can also be used to describe database objects.

DDL is one of the four types of SQL commands, the others being Data Query Language (DQL), Data Manipulation Language (DML), and Data Control Language (DCL). DQL is used to retrieve data from the database, while DML is used to manipulate data present in the database. DCL is used to control access to data and to the database, and TCL (Transaction Control Language) is used to manage transactions in the database.

In summary, DDL is a subset of SQL used to create and modify the structure of database objects in a database, and it is used to define the database schema. It is one of the four types of SQL commands, along with DQL, DML, and DCL.