What is the difference between the stack and the heap?

The stack is a region of memory for automatic (local) variables, managed by the compiler with LIFO allocation and fast, deterministic lifetime tied to scope. The heap (free store) is for dynamic allocation via new/malloc, managed manually (or by smart pointers), with flexible lifetime but slower allocation and risk of leaks.

void f() {
    int a = 10;          // stack
    int* p = new int(20); // heap
    delete p;            // must free heap memory
}