After using Arch for a while and installing and removing applications you are bound to accumulate orphaned packages… which are just packages that were install as dependencies  but now that the original program has been remove they are no longer needed.  An easy way to remove them automatically is to run the following command:

pacman -Rsn $(pacman -Qqdt)

This will do a query of orphan packages and then dump that right into the command to remove them.