How to find out what build version of SharePoint you are running
When you’re applying updates to a large farm, you really need to check the version number of SharePoint running on each server in the farm in order to maintain consistency. You can do this easily by going to CA > Operations > Servers in Farm.
But what do these version numbers actually mean? There’s a great Wiki maintained here that can answer this:
http://www.sharepointdevwiki.com/display/SharePointAdministrationWiki/SharePoint+Versions
This page will show you all the past and current version numbers and their associated KB articles. Very handy when troubleshooting updates or if you want to check the change log between versions.