PKGBUILD is not actually a package, but instead it is a script which tells
makepkg where to get the source from and how to compile and install it.
everything in detail. So, using PKGBUILD one can install packages from source
but still can be handled by the package manager.
Archlinux hosts a public repository, known as Arch User Repository a.k.a. AUR where general PKGBUILD scripts can be hosted so that users of archlinux can install them. Wiki:Arch_User_Repository describes everything about AUR.
Note: Following PKGBUILD(s) are written by me and they may or may not be configured according to my needs. In other words some of the PKGBUILD(s) below are modified versions of the ones hosted on AUR.