Blender uses a right-angled "Cartesian" coordinate system with the Z axis pointing upwards.
- X axis
Left / Right
- Y axis
Front / Back
- Z axis
Top / Bottom
You can select the viewing direction for a 3D Viewport with the View menu entries, Navigation Gizmo, or by pressing the hotkeys.
These operators change the view to be aligned with the specified global axes:
Holding Ctrl shows the other side of the same axis:
Holding Shift aligns the view relative to the active selection. So you can for example, view a rotated objects side, or align the view to the active face in mesh Edit Mode.
The view can also be aligned by holding Alt-MMB and moving the mouse towards the view to align to.