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

1// Task
2public async Task<int> GetValueAsync()
3{
4    return await ComputeAsync();
5}
6
7// ValueTask
8public ValueTask<int> GetValueAsync()
9{
10    if (_cached)
11        return new ValueTask<int>(_value);
12    return new ValueTask<int>(ComputeAsync());
13}