A "ROM" is the image of a game's data files. It doesn't do anything by itself. To use a ROM, you have to have an "Emulator," which is a program designed to emulate, i.e. copy, the functions of a system using your computer. There are emulators that recreate Nintendo, Super Nintendo, Amstrad, Commodore 64, etc. You have to find the one you want, download it, install and configure it, and then you can use ROMs to play games for that system.
The AVP Galaxy ROM pack is merely a package that contains a lot of different Alien and Predator game images from the past 20 years. There's a thread about the pack on the AvP Games forum (which is probably where this post belongs.)
You can find emulators for a lot of different consoles and systems at
www.zophar.net,
www.emuunlim.com,
www.emulator-zone.com, or
www.ngemu.com.
