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 broader set of rows, often referred to as a window. This window can be defined by a partition (dividing the data into groups) and an order (specifying the sequence within each partition). This allows for calculations like running totals, ranking, and calculating moving averages, all within a single query, without the need for subqueries or joins in many cases. They are particularly useful when you need to analyze data within a context of related rows, such as calculating the sales rank of each product within a specific region or finding the top 3 performers in a department. The results of window functions are displayed alongside the original data, making them a valuable tool for data analysis and reporting. Understanding window functions is crucial for creating complex queries that provide insights into data trends and patterns.