We use the following standard orientation to display data:
+z points in superior direction
-x points in left direction (sometimes a bubble or some other artifact in the data may indicate left and right)
-y points in posterior direction
-z points in inferior direction
As the explanation is a bit tricky, here another try: If you look from the front (anterior) your position is at +y and you look into -y direction.
See Wikipedia for more information on anatomical location terms.