C#: What is the difference between Task and ValueTask?

1// Task — always allocates
2public async Task<int> GetDataAsync() {
3    return await httpClient.GetIntAsync("url");
4}
5
6// ValueTask — no allocation if synchronous
7public async ValueTask<int> GetDataAsync() {
8    if (cache.Contains("key")) {
9        return cache["key"];  // No allocation
10    }
11    return await httpClient.GetIntAsync("url");
12}