The difference is that
/* */ can be used in a PL/SQL block, while
REM[ARK] cannot. The following will work in SQL*Plus:
/* comment */
DBMS_OUTPUT.PUT_LINE('Test'); /* comment */
These will not:
DBMS_OUTPUT.PUT_LINE('Test'); REM comment
The 11.2 documentation on all comment types has more comment information. The basics are...
You can enter comments in a script in
using the SQL*Plus REMARK command for single line comments.
using the SQL comment delimiters /*... */ for single or multi-line
using ANSI/ISO (American National Standards Institute/International
Standards Organization) comments - -
for single line comments.
The documentation also includes notes on four places that comments should not be used, but these do not include any further differences.