What is the difference between resistance welding and spot welding?

Per the definition of the Resistance Welding Manufacturers Alliance:
               RESISTANCE WELDING IS THE JOINING OF METALS BY APPLYING
                                                   PRESSURE
                                                AND PASSING
                                                   CURRENT
                                            FOR A LENGTH OF
                                                       TIME
                 THROUGH THE METAL AREA WHICH IS TO BE JOINED

Spot welding is one of the four resistance welding processes as defined by the Resistance Welding Manufacturing Alliance. The four processes are:

           

THE FOUR RESISTANCE WELDING PROCESSES

All four processes generate the heat necessary to bond the material by passing current through the part. The heat is generated by the principals of Joules Law:

Heat is generated by the (current squared) X (resistance) X (time).

 

Reference: RWMA Resistance Welding Manual 4th Edition