monline

First of all, you need the SVN client. On Mac OS X you can get it through Fink or via FinkCommander (install the svn-client package). On Debian (or derivatives) you just install the subversion package as usually (apt-get install subversion).
Th command to download some source from an SVN repository:

"URL" stands for the trunk URL of the repository you want to get, "PATH" stands for the local directory name to fetch the files into.
Of course you can read the complete documentation of the checkout command with issuing svn help checkout.

PS: checkout can be abbreviated to co.