The Right-Hand Rule
From the Blender Manual:
Blender uses a right-angled “Cartesian” coordinate system with the Z axis pointing upwards. Left/Right corresponds to looking along the X axis, Front/ Back along the Y axis, and Top/ Bottom along the Z axis.
Coordinates are usually right-handed.
For right-handed coordinates your right thumb points along the Z axis in a positive Z-direction and the curl of your fingers represents a motion from the first or X axis to the second or Y axis. When viewed from the top or Z axis the system is counter-clockwise.
For left-handed coordinates your left thumb points along the Z axis in a positive Z-direction and the curled fingers of your left hand represent a motion from the first or X axis to the second or Y axis. When viewed from the top or Z axis the system is clockwise.
Interchanging the labels of any two axes reverses the handedness. Reversing the direction of one axis (or of all three axes) also reverses the handedness. (If the axes do not have a positive or negative direction then handedness has no meaning.) Reversing two axes amounts to a 180° rotation around the remaining axis.