Gentoo's overlays are portage's tree extensions. Portage is the big repository of ebuilds of Gentoo to installs all your favorite softwares. Gentoo is pretty well stuffed on this side. But sometime you need an exotic soft unavailable in portage. Some people provide overlay containing softwares that you can add in your Gentoo. To add an overlay, the official way is to use layman. But let's be honest, it can be done without it quite easily. So let's see how to do it.
You just have to edite /etc/portage/repos.conf/gentoo.conf. And add your repo like this :
[nom de l'overlay] priority = 50 location = /where/you/want/to/store/it sync-type = git (or svn or …) sync-uri = git://url.of/overlay.git auto-sync = yes
And now emerge --sync or better eix-sync. You're done
You don't need layman anymore. Isn't this beautiful ?