What is MOSS in MS SharePoint?

To answer the thing in very short: in SharePoint Server2007 following versions available:1.
Free, named "WSS3.0 (Windows SharePoint Services)"2.
Paid, Microsoft Office SharePoint Server2007 (Standard and Enterprise edition) the short form is MOSS In case of SharePoint2010/13 the versions are named as below:1.
Free, Microsoft SharePoint Foundation2010/132.
Paid, Microsoft SharePoint Server2010/13 (this has standard and enterprise edition) I hope this will clarify the things very easily.
Thanks, Ibrahim

Office SharePoint Server (MOSS) is a separately purchased product that offers richer features for business..
Collaboration: allow teams to work together, collaborate on and publish documents, maintain task lists, implement workflows, and share information through the use of wikis and blogs.
Portals: create a personal MySite portal to share information with others and personalize the user experience and content of an enterprise Web site based on the user's profile.
Enterprise Search: quickly and easily find people, expertise and content in business applications.
Enterprise Content Management: create and manage documents, records, and Web content.
Business Process and Forms: create workflows and electronic forms to automate and streamline your business processes.
Business Intelligence: allow information workers to easily access critical business information, analyze and view data, and publish reports to make more informed decisions.