ROWS/RANGE allows you to change the frame size within a partition. Valid arguments for ROWS are:

RANGE is only supported with UNBOUNDED and CURRENT ROW frame delimiters, It cannot be used with N PRECEDING or N FOLLOWING.

ROWS UNBOUNDED PRECEDING:

A simple example to demonstrate use of ROWS is generating cumulative total. Consider following data:

Year        Month       Amount
2011        1           1000.00
2011        2           2000.00
2011        3           3000.00
2011        4           4000.00
2011        5           5000.00
2011        6           6000.00
2011        7           7000.00

To generate a cumulative total we need sum of all rows till current row. This can be done by using UNBOUNDED PRECEDING:

SELECT [Year], [Month],
       SUM([Amount]) OVER (ORDER BY [Year], [Month] ROWS UNBOUNDED PRECEDING)
       AS [Amount]
FROM   dbo.SalesData
GROUP BY [Year], [Month], [Amount]

When no starting or ending position is defined, it defaults to CURRENT ROW. I.e. for Month = 3, Amount is generated as sum of Amount of Month = 1 + Amount of Month = 2 + Amount of Month = 3

Result Set:

Year        Month       Amount
2011        1          1000.00
2011        2          3000.00
2011        3          6000.00
2011        4          10000.00
2011        5           15000.00
2011        6           21000.00
2011        7           28000.00

ROWS BETWEEN N PRECEDING AND CURRENT ROW:

By using N PRECEDING you can specify to start frame N rows before current row. For example, to generate Amount by current month + previous month:

SELECT [Year], [Month],
       SUM([Amount]) OVER (ORDER BY [Year], [Month] ROWS 1 PRECEDING)
       –End default to CURRENT ROW
       –Or you can also write
       –SUM([Amount])
       –OVER (ORDER BY [Year], [Month] ROWS BETWEEN 1 PRECEDING AND CURRENT ROW)
       –To make is more readable
       AS [Amount]
FROM   dbo.SalesData
GROUP BY [Year], [Month], [Amount]

Result set:

Year        Month       Amount
2011        1          1000.00
2011        2           3000.00
2011        3           5000.00
2011        4           7000.00
2011        5           9000.00
2011        6          11000.00
2011        7           13000.00

More combinations to try:

CREATE TABLE dbo.SalesData
(     
       [Year]        INT,
       [Month]       INT,
       [Amount]      MONEY
)
 
INSERT INTO dbo.SalesData VALUES
(2011, 1, 1000.00), (2011, 2, 2000.00), (2011, 3, 3000.00),
(2011, 4, 4000.00), (2011, 5, 5000.00), (2011, 6, 6000.00),
(2011, 7, 7000.00)

SELECT [Year], [Month], [Amount],
       –SUM([Amount]) OVER (ORDER BY [Year], [Month] ROWS 1 PRECEDING)
       –SUM([Amount]) OVER (ORDER BY [Year], [Month] ROWS 1 FOLLOWING)
       –SUM([Amount]) OVER (ORDER BY [Year], [Month]
       –     ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
       –SUM([Amount]) OVER (ORDER BY [Year], [Month]
       –     RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
       –SUM([Amount]) OVER (ORDER BY [Year], [Month] RANGE UNBOUNDED PRECEDING)
              AS [SUM]
FROM   dbo.SalesData
GROUP BY [Year], [Month], [Amount]

Hope This Helps! Cheers!