Viewpoint

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:

Top

Numpad7

Front

Numpad1

Right

Numpad3

Holding Ctrl shows the other side of the same axis:

Bottom

Ctrl-Numpad7

Back

Ctrl-Numpad1

Left

Ctrl-Numpad3

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.