This document is usually the focal point of a conference website. It is built by the general chair of the conference and the link to this page is sent to many academics and conference listing websites (such as the IEEE conference search) in order to promote the conference. Note that the call for papers does not HAVE to be a webpage (ie., it can be a full-page advertisement in a Journal), but these days most conferences use a webpage for convenience.
The call for papers for a conference usually outlines the details of the conference. This includes dates for the conference, details of the place in which it is being held and also things like accomodation and prices. More importantly, the call for papers also makes a list of the topics that are suitable for this conference and establishes a deadline by which papers must be in by. It also lets an author know when they can expect a decision on whether their paper has been accepted or rejected.