Working of window functions and idea window size for window function
The elementary difference is that window functions are applied to all rows in a result set to compute additional columns after the rest of the result set has been determined. No row is dropped. They are available since PostgreSQL 8.4.

The LIMIT and OFFSET clauses of the SELECT command on the other hand do not compute additional columns. They just pick a certain "window" of rows from the result set (in cooperation with the ORDER BY clause) and discard the rest. Have been there like for ever.

While certain tasks can be tackled with either of these tools, they are very different in nature.

For your simple task


  sorting data on date and then bring the latest data first


you don't need either of them. Just add:

ORDER BY date_col DESC




According to your comment, you would need:

SELECT col1, col2
FROM   tbl
ORDER  BY date_col DESC
LIMIT  100   -- 100 latest rows
OFFSET 0;    -- just noise, but may be easier to code


Retrieve more:

...
LIMIT  100
OFFSET 100;  -- next page of 100 rows ...


Be sure to have an index on date_col in either case!