- DDL (Data Definition Language) refers to the CREATE, ALTER and DROP statements
DDL allows to add / modify / delete the logical structures which contain the data or which allow users to access / mantain the data (databases, tables, keys, views...). DDL is about "metadata".
- DML (Data Manipulation Language) refers to the INSERT, UPDATE and DELETE statements
DML allows to add / modify / delete data itself.
- DQL (Data Query Language) refers to the SELECT, SHOW and HELP statements (queries)
SELECT is the main DQL instruction. It retrieves data you need. SHOW retrieves infos about the metadata. HELP... is for people who need help.
- DCL (Data Control Language) refers to the GRANT and REVOKE statements
DCL is used to grant / revoke permissions on databases and their contents. DCL is simple, but MySQL's permissions are rather complex. DCL is about security.
- DTL (Data Transaction Language) refers to the START TRANSACTION, SAVEPOINT, COMMIT and ROLLBACK [TO SAVEPOINT] statements
DTL is used to manage transactions (operations which include more instructions none of which can be executed if one of them fails).