Spark SQL is Apache Spark's interface for working with structured data at scale. It allows querying massive datasets (terabytes to petabytes) using standard SQL syntax while leveraging Spark's distributed computing engine. Under the hood, Spark SQL optimizes queries, parallelizes execution across clusters, and manages memory efficiently. Spark SQL is the foundation for modern data lakes, batch ETL, and large-scale analytics. It's the dominant tool for distributed SQL processing.