The number in parentheses specifies the precision of fractional seconds to be stored. So, (0) would mean don’t store any fraction of a second, and use only whole seconds. The default value if unspecified is 6 digits after the decimal separator.
So an unspecified value would store a date like:
TIMESTAMP 24-JAN-2012 08.00.05.993847 AM
And specifying (0) stores only:
TIMESTAMP(0) 24-JAN-2012 08.00.05 AM
See Oracle documentation on data types.