Can I use Pacman to remove what I installed via yaourt? Or I should use yaourt? Is there a difference?

Not 100% sure, since i don't do this often. But yaourt is a pacman Wrapper, so you could just use yaourt or pacman.

always check the package with pacman -Q to ensure that it is in pacman database, then remove the package as usual.

You can remove it with either pacman or yaourt.

When you install something using yaourt, what you are doing is download the package build and related files, running makepkg, and installing the .pkg.tar.xz with "pacman -U"

Pacman considers packages installed on your system, but not in any of your repos (core, extra, community, testing, ...) as local packages.

You can list local packages with "pacman -Qm". These will be packages installed through yaourt, ones you've downloaded the PKGBUILD on your own and installed, and ones you've created with your own PKGBUILD...

Cool? Might have given you a little more info than you asked for, but I hope for the better.

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