ROLLUP will provide the functional result to each group and entire aggragation of functional result of all groups. ROLLUP is a simple extension to the GROUP BY clause, so its syntax is extremely easy to use. The ROLLUP extension is highly efficient, adding minimal overhead to a query.
For n number of columns in the ROLLUP list, n+1 number of result sets will be produced.

Without ROLLUP:

SELECT PRODUCT_ID, YEAR, SUM(PRICE)
FROM SALES
GROUP BY PRODUCT_ID, YEAR
ORDER BY PRODUCT_ID, YEAR;

PRODUCT_ID	YEAR	SUM(PRICE)
100	2010	5000
100	2011	5000
100	2012	5000
200	2010	9000
200	2011	9000
200	2012	9000
300	2010	7000
300	2011	7000
300	2012	7000
300	2013	7000

With ROLLUP:

SELECT PRODUCT_ID, YEAR, SUM(PRICE)
FROM SALES
GROUP BY ROLLUP(PRODUCT_ID, YEAR)
ORDER BY PRODUCT_ID, YEAR;

PRODUCT_ID	YEAR	SUM(PRICE)
100	2010	5000
100	2011	5000
100	2012	5000
100	<null>	15000
200	2010	9000
200	2011	9000
200	2012	9000
200	<null>	27000
300	2010	7000
300	2011	7000
300	2012	7000
300	2013	7000
300	<null>	28000
<null>	<null>	70000

Partial rollup:
ROLLUP is defined partially in the GROUP BY clause.

SELECT PRODUCT_ID, YEAR, SUM(PRICE)
FROM SALES
GROUP BY PRODUCT_ID, ROLLUP(YEAR)
ORDER BY PRODUCT_ID, YEAR;

PRODUCT_ID	YEAR	SUM(PRICE)
100	2010	5000
100	2011	5000
100	2012	5000
100	<null>	15000
200	2010	9000
200	2011	9000
200	2012	9000
200	<null>	27000
300	2010	7000
300	2011	7000
300	2012	7000
300	2013	7000
300	<null>	28000

In the above example, rollup summary is available for every group but not with the grand total since its defined only for year.