CVS
Using CVS to Upgrade a Drupal Site
If you have many Drupal sites under development, it may become quite difficult dealing with the frequent security upgrades that are released. Learning to use CVS commands can simplify this process considerably. CVS or Concurrent Versions System is a predecessor to Subversion.
As Collins-Sussman wrote in Version Control with Subversion:
“In the world of open source software, the Concurrent Version System (CVS) has long been the tool of choice for version control. And rightly so. CVS itself is free software, and its non-restrictive modus operandi and support for networked operation – which allow dozens of geographically dispersed programmers to share their work – fits the collaborative nature of the open-source world very well. CVS and its semi-chaotic development model have become cornerstones of open-source.”