http://www.orafaq.com/faq/what_are_the_difference_between_ddl_dml_and_dcl_commands

DDL, DML and DCL commands

DDL
Data Definition Language (DDL) statements are used to define the database structure or schema. Some examples:

CREATE - to create objects in the database
ALTER - alters the structure of the database
DROP - delete objects from the database
TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
COMMENT - add comments to the data dictionary
RENAME - rename an object

DML
Data Manipulation Language (DML) statements are used for managing data within schema objects. Some examples:

SELECT - retrieve data from the a database
INSERT - insert data into a table
UPDATE - updates existing data within a table
DELETE - deletes all records from a table, the space for the records remain
MERGE - UPSERT operation (insert or update)
CALL - call a PL/SQL or Java subprogram
EXPLAIN PLAN - explain access path to data
LOCK TABLE - control concurrency

DCL
Data Control Language (DCL) statements. Some examples:

GRANT - gives user's access privileges to database
REVOKE - withdraw access privileges given with the GRANT command

TCL
Transaction Control (TCL) statements are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.

COMMIT - save work done
SAVEPOINT - identify a point in a transaction to which you can later roll back
ROLLBACK - restore database to original since the last COMMIT
SET TRANSACTION - Change transaction options like isolation level and what rollback segment to use

DML commands can't be rollback when a DDL command is executed immediately after a DML. DDL after DML means "auto commit". The changes will return on disk not on the buffer. If the changes return on the buffer it is possible to rollback not from the disk.
 
difference between DDL,DML,TCL and DCL:

DDL COMMANDS:
CREATE,ALTER,DROP AND TRUNCATE are called DDL commands.
They are called Data Definition since they are used for defining the data. That is the structure of the data is known through these DDL commands.

DML COMMANDS:
DML commands are used for data manipulation. Some of the DML commands insert, select, update, delete etc.
Even though select is not exactly a DML language command oracle still recommends you to consider SELECT as an DML command.

TCL:
 For revoking the transactions and to make the data commit to the database we use TCL. Some of the TCL commands are:
1. ROLLBACK
2. COMMIT

ROLLBACK is used for revoking the transactions until last commit.
COMMIT is used for commiting the transactions to the database.
Once we commit we cannot rollback. Once we rollback we cannot commit.
Commit and Rollback are generally used to commit or revoke the transactions that are with regard to DML commands.

DCL:
Data Control Language is used for the control of data. That is a user can access any data based on the priveleges given to him. This is done through DATA CONTROL LANGUAGE. Some of the DCL Commands are:
1. GRANT
2. REVOKE.

SQL statements are often divided into three categories:

DML (Data Manipulation Language). These SQL statements are used to retrieve and manipulate data. This category encompasses the most fundamental commands including DELETE, INSERT, SELECT, and UPDATE.
DML SQL statements have only minor differences between SQL variations. DML SQL commands include the following:
DELETE to remove rows.
INSERT to add a row.
SELECT to retrieve row.
UPDATE to change data in specified columns.

DDL (Data Definition Language). 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. DML 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.

DCL (Data Control Language). These SQL statements control the security and permissions of the objects or parts of the database(s).
DCL SQL statements are also more part of the DBMS and have large differences between the SQL variations. DML SQL commands include the following:
GRANT to allow specified users to perform specified tasks.
DENY to disallow specified users from performing specified tasks.
REVOKE to cancel previously granted or denied permissions.