I undrstand that a trunk is the "main" branch, and understand the concept of branches and tags. but where does the "head" comes into the picture? It seems it is the "latest" revision? if so than what is it's difference to a "trunk"
I'm sure I'm probably mixing unrelated fields, e.g. trunk is to branch as head is to tags?
I dug into some documentations, but I must admit that it wasn't that clear to me time limiting.
HEAD means the most recent revision of them all. Trunk it the branch where most development happens (depends on branching strategy).
For more revision keywords see svnbook. It also describes repository layout