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What does the ROW_NUMBER function do, and what is a common practical use case for it? What is the difference between RANK, DENSE_RANK, and ROW_NUMBER when there are tied values in the ordering column? How do LAG and LEAD window functions let you access values from a previous or following row without needing a self join? How does the PARTITION BY clause within a window function differ from GROUP BY, and why is this distinction useful? How does the ROWS versus RANGE frame specification within an OVER clause affect the result of a window function like a running total? How would you use window functions together to build a comprehensive sales analytics report showing running totals, rankings, and period over period comparisons all in a single query?
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What is a window function in SQL Server, and how is it different from a regular aggregate function used with GROUP BY?

Beginner

Answer

A window function performs a calculation across a set of related rows, similar to an aggregate function, but unlike GROUP BY, it does not collapse those rows into a single summarized result, instead returning a value for every individual row while still having access to information from the related rows around it.
SELECT OrderId, CustomerId, Amount,
  SUM(Amount) OVER (PARTITION BY CustomerId) AS CustomerTotal
FROM Orders;
Real-world example A sales report shows every individual order alongside that customer's overall total spending, using a window function to calculate the total without collapsing the individual order details the way a regular GROUP BY would.

Common follow-ups: What does the OVER clause actually control in a window function?;Can window functions be combined with regular aggregate functions in the same query?

Aggregate Functions & GROUP BY;Common Table Expressions (CTEs)
All 7 Window Functions questions