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The big idea behind window functions is to allow you to process several values of the result set at a time. you see through the window some peer rows and are able to compute a single output value from them, much like when using an aggregate function.
When looking through a window we only see a frame at a time. Its all about the frame.
sachi=# select region, rank() over (W) from sachi.sales WINDOW W as (order by year);
region | rank
--------+------
asia | 1
usa | 2
(2 rows)