What is DDL (Data Definition Language)?
Operations on a database are divided into DDL and DML. 

DDL are operations that alter the structure of the database. This includes creating/dropping tables, constraints, indexes, packages, views, dblinks, adding or removing columns to a table, changing object names and so on.

DML are the operations that alter the data stored in the database. This includes inserting, deleting and updating rows to/from a table.