What is the CLR, and why is it central to how .NET applications execute?

The Common Language Runtime (CLR) is .NET's managed execution engine -- it takes the compiled intermediate language, JIT-compiles it into native machine code on demand, and simultaneously provides the services that make .NET applications robust: automatic memory management via the garbage collector, type safety enforcement, exception handling, and security sandboxing -- without the CLR, none of the automatic, 'managed' behavior C# developers rely on day to day would exist.

class Program {
    static void Main() {
        var list = new List<int> { 1, 2, 3 };
        // The CLR is responsible for: JIT-compiling this method, allocating 'list' on the
        // managed heap, verifying type safety of the List<int> operations, and eventually
        // reclaiming that memory via the garbage collector once 'list' is no longer reachable
    }
}