C++: What is the difference between std::priority_queue and std::queue?

1// priority_queue — largest first
2std::priority_queue<int> pq;
3pq.push(3);
4pq.push(1);
5pq.push(2);
6pq.top();  // 3 (largest)
7
8// queue — first in, first out
9std::queue<int> q;
10q.push(3);
11q.push(1);
12q.push(2);
13q.front();  // 3 (first)