Lower pairs are those for which area contact may be visualized, as in pin connections, crossheads, ball-and socket joints and some others; the relative motion of coincident points of the elements, and hence of their links, are similar, and an exchange of elements from one link to the other does not alter the relative motion of the parts as it would with higher pairs.In kinematics, the two connected physical objects, forming a kinematic pair, are called 'rigid bodies'.
A lower pair is one in which there occurs a surface or area contact between two members, e.g. nut and screw, universal joint used to connect two propeller shafts.
Similarly, the contact between the involute curves that form the meshing teeth of two gears are cam joints, as is a wheel rolling on a surface.
Serial manipulators, like the SCARA robot, connect a moving platform to a base through a single chain of links and joints.
Multiple serial chains connect the moving platform to the base of parallel manipulators, like the Gough-Stewart mechanism.
Topology refers to the arrangement of links and joints forming a manipulator or robot.