We use ROLLBACK TO SAVEPOINT statement to undo the effect of a transaction upto the SAVEPOINT mentioned in ROLLBACK statement.
RELEASE SAVEPOINT is simply used to delete the SAVEPOINT with a name from a transaction. There is commit or rollback for SAVEPOINT in RELEASE statement.
In both the cases we should have first created a SAVEPOINT. Else we will get the error while doing ROLLBACK or RELEASE of a SAVEPOINT.