BEGIN [ WORK | TRANSACTION ]
By default, PostgreSQL executes transactions in unchained mode (also known as "autocommit" in other database systems). In other words, each user statement is executed in its own transaction and a commit is implicitly performed at the end of the statement (if execution was successful, otherwise a rollback is done). BEGIN initiates a user transaction in chained mode, i.e., all user statements after BEGIN command will be executed in a single transaction until an explicit COMMIT or ROLLBACK. Statements are executed more quickly in chained mode, because transaction start/commit requires significant CPU and disk activity. Execution of multiple statements inside a transaction is also useful to ensure consistency when changing several related tables: other clients will be unable to see the intermediate states wherein not all the related updates have been done.
The default transaction isolation level in PostgreSQL is READ COMMITTED, wherein each query inside the transaction sees changes committed before that query begins execution. So, you have to use SET TRANSACTION ISOLATION LEVEL SERIALIZABLE just after BEGIN if you need more rigorous transaction isolation. (Alternatively, you can change the default transaction isolation level; see the PostgreSQL Administrator's Guide for details.) In SERIALIZABLE mode queries will see only changes committed before the entire transaction began (actually, before execution of the first DML statement in the transaction).
Transactions have the standard ACID (atomic, consistent, isolatable, and durable) properties.
START TRANSACTION has the same functionality as BEGIN.
Refer to LOCK for further information about locking tables inside a transaction.
If you turn autocommit mode off, then BEGIN is not required: any SQL command automatically starts a transaction.
BEGIN is a PostgreSQL language extension. There is no explicit BEGIN command in SQL92; transaction initiation is always implicit and it terminates either with a COMMIT or ROLLBACK statement.
Note: Many relational database systems offer an autocommit feature as a convenience.
Incidentally, the BEGIN keyword is used for a different purpose in embedded SQL. You are advised to be careful about the transaction semantics when porting database applications.
SQL92 also requires SERIALIZABLE to be the default transaction isolation level.