A Data Definition Language is a computer language for defining data. XML Schema is an example of a pure DDL (although only relevant in the context of XML). A subset of SQL's instructions form another DDL.
For example in Oracle the DDL statements refer to CREATE,DROP,ALTER etc..
These SQL statements define the structure of a database, including rows, columns, tables, indexes, and database specifics such as file locations. DDL SQL statements are more part of the DBMS and have large differences between the SQL variations. DDL SQL commands include the following:
Create - To make a new database, table, index, or stored query.
Drop - To destroy an existing database, table, index, or view.
DBCC (Database Console Commands) - Statements check the physical and logical consistency of a database.