Joint mobilization is a type of passive movement of a skeletal joint. It is usually aimed at a 'target' synovial joint with the aim of achieving a therapeutic effect.
Mobilization is a manual therapy intervention and is classified by five 'grades' of motion, each of which describes the range of motion of the target joint during the procedure. 
Roman numerals are generally used in labelling the grades of motion (i.e. Grades I to V). Grade V is the same as manipulation.
Mechanisms of action
The different grades of mobilization are believed to produce selective activation of different mechanoreceptors in the joint:
Grade I - Activates Type I mechanoreceptors with a low threshold and which respond to very small increments of tension. Activates cutaneous mechanoreceptors. Oscillatory motion will selectively activate the dynamic, rapidly adapting receptors, i.e. Meissner's and Pacinian Corpuscles. The former responds to the rate of skin indentation and the latter respond to the acceleration and retraction of that indentation.
Grade II - Similar effect as Grade I. By virtue of the large amplitude movement it will affect Type II mechanoreceptors to a greater extent.
Grade III - Similar to Grade II. Selectively activates more of the muscle and joint mechanoreceptors as it goes into resistance, and less of the cutaneous ones as the slack of the subcutaneous tissues is taken up.
Grade IV - Similar to Grade III. With its more sustained movement at the end of range will activate the static, slow adapting, Type I mechanoreceptors, whose resting discharge rises in proportion to the degree of change in joint capsule tension.
Grade V - This is the same as joint manipulation. Use of the term 'Grade V' is only valid if the joint is positioned near to its end range of motion during joint manipulation. Evans and Breen  recently contested this assumption, in fact arguing that an individual synovial joint should be positioned near to its resting, neutral position.