The Rotation Problem

Rotations are made along the length of the joint,in world order, sometimes you have to use the "orient joint" tool to move the axes and have them the same direction, so you do not get problems with joints going one way, and another going differently.Eular rotations are the most common,it requires an rotation order to work, but also can experience problems, such as Gimbal lock, which is when two axes align, and it cannot be moved, eular rotations are often referred to gimbal rotations because of this. 

Another rotation is Quaternion , which uses another axis, "t", which is it's roll value.
In Eular rotations their is an order (x, y and z) that the computer calculates the rotation of an object. There are six combinations which each , place the object in complete different areas in the world space.
xyz, xzy, yzx, yxz, zyx, zxy

If Rotation orders are incorrect these can break the rig, and cause gimbal problems while animating.
Various constraints and ik handles can be applied to joints to create more life like movements, and not requiring to move every joint, controllers can be made to move an whole leg for example,in the reverse foot lock case.