What is a Trie and when would you use it?

A Trie (prefix tree) is a tree-like structure for storing strings where each node represents a character, and paths from root represent prefixes. Each node has up to 26 children (for lowercase letters) and a flag indicating word end. Operations (insert, search, prefix search) are O(m) where m is string length. Ideal for autocomplete, spell checkers, IP routing, and when many strings share common prefixes.