Interview Question for Software Engineer at Yahoo:

To explain them, not just list them, would take a short amount of time. The only difference between In-order, Post-order, and Pre-order is where the node is printed in the function.
In order:
traverse(node.left_chld)
print(node.value)
traverse(node.right_child)

post-order:
traverse(node.left_chld)
traverse(node.right_child)
print(node.value)

pre-order:
print(node.value)
traverse(node.left_chld)
traverse(node.right_child)

level order is what is called breadth first search. Ideally you would use a queue and you'd do something like:

levelorder(root)
  q = empty queue
  q.enqueue(root)
  while not q.empty do
    node << q.dequeue()
    visit(node)
    if node.left != null
      q.enqueue(node.left)
    if node.right != null
      q.enqueue(node.right)

(adapted the last snippet from Wikipedia)