What is the difference between Visa and Interac?

VISA is a card scheme. They work with banks around the world for issuing of VISA branded cards/products and also do acquiring (in a simplistic sense). They are able to do this by extending and reaching out the power of their global VISANet and connecting all the banks, payment processors and payment gateways to it.

Interac, is a Canadian closed loop network (or switch) between all the major Canadian banks to allow inter-bank funds transfers between members.