What is an Extranet

What is an Extranet?

An Extranet is an intranet which extends beyond the physical boundaries of a corporation.

Extranets give access to vendors and suppliers, dealers and distributors to a firm's intranet. Companies include them in order to facilitate the transfer of information. This is information which not accessible to the general public. For example: inventory levels, delivery times, pricing policies, etc.

I understand what an Intranet is. What is a protocol?