There are many source control systems like Git, Clearcase, Subversion (aka SVN), etc, and SVN is a very popular open source source control system. You can use it via its command line commands or via GUI based client tools like TortosieSVN.
Technically all three i.e. trunk, branch and tag are folders in SVN. If you are using tortoise SVN, a popular windows client for subversion, you can explore trunk, branch or tag. If you browse SVN repository using Repo browser or simply open it on any browser e.g. Internet explorer, you will generally see three directories as trunk, branch and tags at root of project. This is actually one of SVN best practice to create this kind of directory structure. As I said earlier, trunk is place where main development happens, and branches are places where different developer work on different functionalities. This division is purely based on how programmer uses trunk and branches. Similarly, tags are used to backup releases e.g. alpha release or beta release or any version of release. Main difference between branch and tag in subversion is that, tag is a read only copy of source code at any point and no further change on tag is accepted, while branch is mainly for development. Other source control like CVS doesn't allow modification on tags but SVN allows changes on tags, which is considered as bad practice. You should not be making any change on tag once created, it should be treated as read only copy of source code only for restore purpose.
- A trunk in SVN is main development area, where major development happens. Like a tree, trunk is a tree’s central superstructure. All branches come out of the trunk.
- A branch in SVN is sub development area where parallel development on different features happens. Branches are created for adding new features/enhancements, bug fixes, and maintenance. After completion of a functionality, a branch is usually merged back into trunk. Tools like TortoiseSVN and IDE plugins for SVN simplifies the code merging task.
- A tag in SVN is a read only copy of source code from branch or trunk at any point of time. A tag is mostly used to create a copy of released source code so that it can be rolled back.
- A head is the latest version in the repository. That is either in the trunk or branch.
Note: That's all on difference between trunk, branch and tags in Subversion or SVN. They are mostly categorized based upon there usage. Though trunk, branch and tag are quite common words in source control, and other SCM also uses these words, there behavior may be little different on other SCM, so don't forget to check SCM documentation.