What is Window Function?
A window function calculates a value across a set of related rows, such as a running total or a rank, without collapsing those rows into one.
Window Function explained
Ordinary aggregates with GROUP BY return one row per group. A window function keeps every row and adds a calculated column beside it, for example each order together with the customer's running total.
The OVER clause defines the window: how rows are grouped (PARTITION BY), in what order, and which neighbouring rows count (the frame).
Example
SUM(amount) OVER (ORDER BY order_date) adds a running total column while still listing every order.
How it differs from similar terms
GROUP BY with an aggregate collapses rows; a window function keeps them.