Salesforce.com's Content Management System (CMS) that allows for the creation, maintenance, and publishing of websites with some integration with Salesforce data. Site.com was originally known as Siteforce.
Questions about Site.com should have this tag.
Site.com does not support Visualforce pages. Salesforce.com offers Force.com Sites which can be used to publish websites backed by Visualforce; however, Force.com Sites is not a CMS.
The Site.com developer force wiki has a suggested Site.com learning path and more resources and links helpful for learning more about Site.com.