A transaction is a logical unit of work performed by a Data Manipulation Language (DML) operation. It contains one or more SQL statements (comprising an atomic unit). A transaction's effects can be committed (applied) using a COMMIT statement or rolled back (undone) using a ROLLBACK statement. Some relational database management systems (RDBMS) automatically commit changes when a DML statement is run in client software. In general, Oracle requires a user to explicitly commit or rollback DML changes. An explicit commit occurs when the COMMIT keyword is used. An implicit commit occurs upon normal termination of an application or when a DDL statement is executed.
It is a good idea to commit changes whenever you have completed any DML. If you do not, there is the potential for database locks to be held which will cause other database sessions to "hang" while waiting for the transaction to complete.