Limits of computation

Describe what a heuristic method is.

Describe what an intractable problem is.

Why is it that Dijkstra's shortest path algorithm can be described as accurate, optimal and complete?

Under which circumstances is it possible to solve an intractable problem with a computer?

What do these time complexities all have in common O(1), O(log n), O(n), O(n log n) & O(n2)