What is a query execution plan and how do you read it?

An execution plan shows how the database executes a query - operations, order, costs, and data access methods. View with EXPLAIN (MySQL/PostgreSQL) or SET STATISTICS IO (SQL Server). Key elements: Seq Scan vs Index Scan, Join types (nested loop, hash, merge), estimated rows, cost/actual time. Read bottom-up or inside-out (innermost operations first). Use to identify full table scans, missing indexes, inefficient joins. Essential for query optimization.