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The 'trunk' is the main line of development in a SVN repository.

A 'branch' is a side-line of development created to make larger, experimental or
disrupting work without annoying users of the trunk version. Also, branches can
be used to create development lines for multiple versions of the same product,
like having a place to backport bugfixes into a stable release.

Finally, 'tags' are markers to highlight notable revisions in the history of the
repository, usually things like "this was released as 1.0".

See the HTML version of "Version Control with Subversion", especially Chapter 4:
Branching and Merging or buy it in paper (e.g. from amazon) for an in-depth
discussion of the technical details.
http://svnbook.red-bean.com/
http://svnbook.red-bean.com/en/1.5/svn.branchmerge.html

http://stackoverflow.com/questions/698313/what-is-trunk-branch-and-tag-in-subversion