C#: What is the difference between ref and out?

1// ref — must be initialized
2void Increment(ref int x) {
3    x++;
4}
5
6int a = 5;
7Increment(ref a);  // OK
8
9// out — no initialization needed
10bool TryParse(string s, out int result) {
11    result = int.Parse(s);
12    return true;
13}
14
15int number;
16TryParse("123", out number);  // OK