What is a trie and when is it useful?

class Trie:
    def __init__(self):
        self.root = {}

    def insert(self, word):
        node = self.root
        for ch in word:
            node = node.setdefault(ch, {})
        node["$"] = True              # end-of-word marker

    def search(self, word):
        node = self._walk(word)
        return node is not None and "$" in node

    def starts_with(self, prefix):
        return self._walk(prefix) is not None

    def _walk(self, s):
        node = self.root
        for ch in s:
            if ch not in node: return None
            node = node[ch]
        return node


# --- demo ---
t = Trie()
for w in ["cat", "car", "can"]:
    t.insert(w)
print(t.search("car"))        # True
print(t.search("ca"))         # False (prefix, not a full word)
print(t.starts_with("ca"))    # True