What are SQL window functions and how are they used?

Window functions perform calculations across rows related to current row without collapsing rows (unlike GROUP BY). Syntax: function() OVER (PARTITION BY cols ORDER BY cols). Functions include: ROW_NUMBER(), RANK(), DENSE_RANK() for ranking; SUM(), AVG() for running totals; LAG(), LEAD() for accessing adjacent rows; FIRST_VALUE(), LAST_VALUE() for boundary values. Example: ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC) ranks employees within each department.