Subversion is an open source version control system, it manages files and directories over time. It is like an ordinary file server, which records each and every change made to it.
The central repository has all the data in predefined format i.e. a tree structure , this allows the user to see all the changes made to it any time. And also user can recover any previous version of that project code.
This tutorials explains everything you need to know about SVN merge. This explains the basics of merging, how to merge two branches, how to merge with trunk, and reintegrating a branch using examples.