What are select options in SAP? Difference between parameters and select-options?

select options provide ranges where as parameters do not.

SELECT-OPTIONS declares an internal table which is automatically filled with values or ranges of values entered by the end user. For each SELECT-OPTIONS , the system creates a selection table.
SELECT-OPTIONS <SEL> FOR <field>.

A selection table is an internal table with fields SIGN, OPTION, LOW and HIGH.
The type of LOW and HIGH is the same as that of <field>.
The SIGN field can take the following values: I Inclusive (should apply) E Exclusive (should not apply)

The OPTION field can take the following values: EQ Equal GT Greater than NE Not equal BT Between LE Less than or equal NB Not between LT Less than CP Contains pattern GE Greater than or equal NP No pattern.

diff:-
PARAMETERS allow users to enter a single value into an internal field within a report.
SELECT-OPTIONS allow users to fill an internal table with a range of values.