downloading

hi

very pleased with my new imac just a bit confused as to what to do with apps that r downloaded to my desktop which r viewed as .dmg and extra drives. do i just copy these .dmg files to my apps folder or delete them and do the drives always appear on the desktop.
thanks
chris

Normally, I copy the app out of the dmg to some folder (unless it has its own installer) and then after "ejecting" the .dmg, I store it in another folder for future burning to CD. That way if something happens, I will have a backup and won't have to download the dmg file again.