The COMMIT statements end the present transaction and make permanent any changes made during that transaction. Till you commit the changes, other users cannot access the changed data; they see the data as it was before you made the changes.
Consider a simple transaction which transfers money from one bank account to the other. The transaction needs two updates as it debits the first account, and then credits the second. In the illustration below, after crediting the second account, you issue a commit that makes the changes everlasting. Only then do other users see the changes.
UPDATE accts SET bal = my_bal - debit
WHERE acctno = 7715;
UPDATE accts SET bal = my_bal + credit
WHERE acctno = 7720;
The COMMIT statements release all row and table locks. It also erases any savepoint marked as the last commit or rollback. The elective keyword WORK has no effect other than to get better readability. The keyword END signals the end of the PL/SQL block, not the end of the transaction. Now as a block can span a multiple transactions, the transaction can cover multiple blocks.
The COMMENT clause specifies a comment to be related with the distributed transaction. If you issue a commit, the changes to each database affected by a distributed transaction are made permanent. Though, if a network or machine fails during a commit, the state of the distributed transaction may be unknown or in doubt. In that situation, the Oracle stores the text specified by the COMMENT in the data dictionary beside with the transaction ID. The text should be a quoted literal up to 50 characters in size.