C++: What is the difference between std::map and std::unordered_map?

1// map — sorted
2std::map<int, std::string> m;
3m[3] = "three";
4m[1] = "one";
5// Iterates: 1, 3
6
7// unordered_map — hash
8std::unordered_map<int, std::string> um;
9um[3] = "three";
10um[1] = "one";
11// Iterates: 3, 1 (any order)