C#: What is pattern matching in C#?

1if (obj is string s)
2    Console.WriteLine(s.Length);
3
4if (obj is not null)
5    Process(obj);

1if (person is { Age: > 18, Name: not null })
2    ProcessAdult(person);

1var result = shape switch
2{
3    Circle { Radius: > 0 } c => c.Area,
4    Rectangle { Width: var w, Height: var h } => w * h,
5    _ => 0
6};

1static string Classify(int value) => value switch
2{
3    < 0 => "negative",
4    0 => "zero",
5    > 0 and < 100 => "small",
6    >= 100 => "large"
7};

1if (score is >= 90 and <= 100)
2    Console.WriteLine("A");