emulators are programs which emulates other devices on the PC. for example: you can get emulator for PS1 like CVGS and then insert the PS1 CD in the CD rom and start the emulator. wooowwwwww now you can PS1 games on your PC.
you can also install virtuall drives program and then make a virtuall disk of favourite game then just insert that virtuall disk into virtuall drive and play the game without the real game CD