C++: What is emplace_back vs push_back?

1#include <vector>
2
3class Expensive {
4    std::string data;
5public:
6    Expensive(const std::string& s) : data(s) {
7        std::cout << "Constructed\n";
8    }
9    Expensive(const Expensive& other) : data(other.data) {
10        std::cout << "Copied\n";
11    }
12    Expensive(Expensive&& other) noexcept : data(std::move(other.data)) {
13        std::cout << "Moved\n";
14    }
15};
16
17std::vector<Expensive> vec;
18
19vec.push_back(Expensive("hello"));  // Construct + Move
20vec.emplace_back("hello");           // Construct only
21
22// emplace_back with complex arguments
23vec.emplace_back(std::string(10, 'a'));  // Constructs in-place