What's the difference between T, volatile T, and std::atomic

Given the following sample that intends to wait until another thread stores 42 in a shared variable shared without locks and without waiting for thread termination, why would volatile T or std::atomic<T> be required or recommended to guarantee concurrency correctness?

#include <atomic>
#include <cassert>
#include <cstdint>
#include <thread>

int main()
{
  int64_t shared = 0;
  std::thread thread([&shared]() {
    shared = 42;
  });
  while (shared != 42) {
  }
  assert(shared == 42);
  thread.join();
  return 0;
}

With GCC 4.8.5 and default options, the sample works as expected.