C#: What is the difference between ConcurrentDictionary and Dictionary?

1// ConcurrentDictionary
2var dict = new ConcurrentDictionary<string, int>();
3dict.TryAdd("key", 1);
4dict.AddOrUpdate("key", 2, (k, v) => v + 1);
5
6// Dictionary (with lock)
7var dict = new Dictionary<string, int>();
8lock (dict) { dict["key"] = 1; }