If you already have Psychtoolbox, you already have SVN. Otherwise follow step 1 & 2 on these instructions for installing SVN.
Subversion (abbreviated SVN) is the name of a program that allows many people at once to work on a single programming project. Each project in SVN has a single place - a repository - where all the files are stored. These repositories are hosted by websites such as sourceforge.net and xp-dev.comand are often free.
When a user wants to work on a project maintained in SVN, they can issue a single command to the repository and have the entire project downloaded to a local folder. The user can then use, add, change or delete files in this local directory.
The main functionality of SVN comes up when a user wants to have the changes they made to their local files reflected in the main repository for everyone else to use. SVN can identify which files have been modified and upload only those changes to the repository. SVN can also identify any files that have been changed in the repository by other users and download only those changes to update the local files. In short, SVN offers a very simple way to keep multiple people up to date on a constantly evolving project.
A great use of SVN is to share and update the library of matlab functions used for analysis. A single matlab script (see below) is capable of accessing the repository and setting a user up with the entire library for the first time, without having to understand any of the SVN commands themselves. Even better, the same script can be run whenever updates are made to the library.
Another use of SVN is for sharing an experiment or analysis project between multiple
people. For example, I can develop a project and have other labs
download it through SVN. If they request any changes to the project,
they can be developed and tested locally as usual, and then easily
updated for the other labs. After the other labs collect data and
perform analysis, the data files as well can be transferred back to me
through SVN. All this without zipping up files, emailing large archives,
and being careful not to overwrite the wrong files.
If you are interested in knowing more,
there is a great tutorial and a full reference book.