Use two different rotation modes to describe the an object’s rotation in the scene. Pick the mode that gives you the most flexibility.
When rotating a node in your scene, you may have noticed two modes – or ways to describe – rotation are available. Each is better suited to different rotation tasks.
Select any scene node that supports rotation (meshes, global lights, viewpoints, connectors, groups, etc.), and the properties box will show two rotation modes
Mode  Useful For  Example in a Car Model 

Euler 
simple rotations, often those aligned with one of the 3 axes 
Turn a wheel around the X axis to roll the car forward. Turn a wheel around the Y axis to steer the car left or right. 
Axis and Angle 
complex rotations that aren’t aligned with one of the axes 
Turn a steering wheel left and right, but the wheel is on a steering column 49 degrees from vertical. 
Euler mode rotation is simple.
The rotate gizmo for Euler looks like three rings orbiting the node.
Drag the 3 rings to define how far your mesh rotates around any of the 3 axes.
Axis and Angle mode rotation is a two step process.
The rotate gizmo for Axis and Angle looks like a sphere with a ring around it and an arrow going through it.

First, define the axis your node will rotate around.
Drag the sphere to aim the arrow in any direction. This arrow is the rotation axis. The mesh will rotate around this arrow like a wheel around an axle.

Then, define how far the node will turn around that axis.
Drag the ring around the sphere. The farther you drag the ring, the farther the mesh rotates around the axis.
Why are two rotation modes available?
Euler rotations are easy to understand, but can sometimes have a problem. Since the 3 rotational axes are nested (like a gimbal or gyroscope), it’s possible to rotate two of the rings so they are in the same plane. This is called “gimbal lock”, because rotation becomes limited to 2 dimensions instead of 3. Because of this, translating  or animating  the rotated mesh from this state results in a warped, strange jump as the lock resolves.
Which mode is best?
Both modes are just as accurate, and both can be manipulated with Snap code.
It's easy to switch between modes. When you change the rotation mode on a mesh, your previous settings are not lost: any rotation you define in one mode is automatically translated into the other mode when you change the mode in the properties expander.
Most scene designers keep their goal for rotating the mesh in mind, and simply use the technique which best suits the situation.
Don't forget groups.
Group your meshes to factor complex positioning into simpler tasks. For example, complex rotations can often be created by nesting a mesh within one or more groups, and rotating each group. The scene will do the math to summarize all the rotations onto the contained meshes.
In the example below, a moon orbits a planet. They both are in an invisible group, and that group containing them then orbits an Epicor logo. The complex path of the moon around the planet around the logo is calculated automatically, from the two simpler rotations.