This aim of this note is to provide the shortest and simplest explanation of the concepts of trunk and HEAD in a Version Control System like CVS and Subversion (SVN.)
Like in botany, trunk is a tree’s central superstructure. All branches come out of the trunk:

trunk and branches
Main development is performed in the trunk. Releases are branched for bug fixes and maintenance releases.
In the next diagram, the tip of the arrows for trunk and a branches are HEADs. Each branch and trunk have a HEAD.

HEADs on trunk and branches
That’s it!
