Everybody can write here, but only the owner of the file may remove the file. Root may do it too, of course. This directory has a lot of subdirs. In local you can place local extensions of the system, and if you upgrade your system the upgrade shouldn't do anything about local, so your changes will still be there. This directory holds files that could be a lot bigger in a short time, and get smaller really fast too.
Typical examples are mail and news spools.
Well, that's about it.