Window functions are a powerful tool in SQL that allow you to perform calculations over a set of rows related to the current row, without grouping the data. Unlike aggregate functions (like SUM, AVG, COUNT), which summarize data across groups, window functions operate on a window of rows, providing insights into the context of each row. They're incredibly useful for tasks like calculating running totals, ranking records, and partitioning data for analysis. For instance, you might want to see how a sales figure for a particular day compares to the same day in the previous week or month. Window functions provide the framework for this type of analysis. A key distinction is that window functions don't change the shape of the result set; they add calculated columns to the existing rows. This makes them ideal for tasks that require context-sensitive calculations without losing the individual row details.