What is copy-on-write and how is it used in fork()?

Copy-on-write (COW) is an optimization that delays copying until modification. In fork(), instead of duplicating entire address space, parent and child share same physical pages marked read-only. When either writes, a page fault triggers copying of that page only. Benefits: fast fork() (just copy page tables), memory efficient (shared read-only data like code), enables efficient fork-exec pattern where child immediately replaces memory with exec().