According to Wikipedia Cloud computing is the delivery of computing as a service rather than a product, whereby shared resources, software, and information are provided to computers and other devices as a utility (like the electricity grid) over a network (typically the Internet).
Fog Computing is a paradigm that extends Cloud computing and services to the edge of the network. Similar to Cloud, Fog provides software, information, and application services to end-users. What is different about Fog Computing is its proximity to end-users, its dense geographical distribution, and its support for mobility.
Virtual Telephony Application Grid (V-TAG) is 3CLogic’s implementation of Fog Computing for delivery of contact center services over the cloud. Its similarity with cloud computing is that contact center services are delivered like a utility over the internet. Its similarity with Fog Computing is that in V-TAG contact center services are hosted and processed at the network edge or even on end devices such as PCs. Broadly speaking V-TAG implements edge computing for voice, chat, email, and SMS traffic in contact centers as opposed to processing these services in centralized switches and servers.