C++: What is the difference between const and constexpr?

1const int x = 10;
2constexpr int y = 10;
3
4int arr1[x];  // Error (VLA)
5int arr2[y];  // OK
6
7constexpr int square(int n) {
8    return n * n;
9}
10int arr3[square(5)];  // arr3[25]