To add to Bob's post, when you install Flash, it comes on a .dmg file, which is a disk image. It gets mounted as a drive when you click on it, and the installer is then on the mounted drive. Assuming you've completed the installation, you can unmount the drive by either using Disk Utility, or by dragging the icon for it to the trashcan. Then once it's unmounted, you can delete the .dmg file that was downloaded.