What are closures in JavaScript and how are they used?

A closure is a function that remembers variables from its outer (enclosing) scope even after the outer function has returned. The inner function maintains a reference to the outer scope's variables. Uses: data privacy (module pattern), function factories, callbacks with state, memoization, partial application/currying. Example: counter factory where inner function accesses and modifies outer variable. Closures enable functional programming patterns. Caution: closures in loops can cause issues (use let or IIFE). Understanding closures is essential for advanced JavaScript patterns.