We use the following standard orientation to display data:
- +x points in right direction (sometimes a bubble or some other artefact may indicate left and right)
- +y points in anterior direction
- +z points in superior direction
- -x points in left direction (sometimes a bubble or some other artefact 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.