The stack is a tightly organized, last-in-first-out region used for function calls — local variables get pushed on when a function starts and popped off automatically when it returns, which makes it extremely fast but limited in size. The heap is a much larger, loosely organized pool used for data whose size or lifetime isn't known in advance; you request a chunk of it manually (or a garbage collector does), and it has to be tracked and freed explicitly or reclaimed later. The tradeoff is speed and simplicity (stack) versus flexibility (heap).