AUR is an unofficial arch repository where you can find many packages, though the problem is that if you don't have everything under the scope of pacman your OS may crash. A good practice is to build AUR packages using pacman.
Let say we want to install openframeworks from AUR. You have to do the following steps:
$HOME, lets say
mkdir $HOME/PKGBUILDS, and
wget https://aur.archlinux.org/cgit/aur.git/snapshot/openframeworks.tar.gz(the copied link location, usually in terminal use
tar xf <package name>.tar.gzto uncompress the package folder
makepkgto build the package. If no errors/warnings occur you should be OK.
sudo pacman -U $HOME/PKGBUILDS/package_name-version.pkg.tar.xz
If you would like to remove the package
sudo pacman -R <package name>.