Version Control
Version Control is sometimes internal to a software application or tool. Users of OpenOffice Writer will be familiar with how you can open a document sent by a colleague, turn on "track changes" and then your edits to the document will be clearly shown so that you can send the document back to the orginal author to provide feedback and the opportunity to merge changes in a friendly, efficient manner.
Version Control in a software development environment is a large part of the infrastructure that makes software development possible -- because it provides the same feedback and opportunity to merge changes in a friendly, efficient manner. The tools that software developers use are CVS, Subversion, Mercurial and Git or others.
For distributed version control, merging and merge tracking including incorporation of "Third-party code" into your software products is really difficult or impossible with Subversion yet possible and even fun with Git.
