What is the difference of DVD-RAM and DVD-ROM?

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I encountered a problem when I installed a game (COD4-Modern Warfare). I ran the set-up and copied what must be copied. And everthing seemed ok. After the installation wizard finished, I was excited to play the game. To my suprise, as entered to the game, there was an error saying "Please insert the right DVD-ROM. and Click ok and restart application". I can't understand what is happening. I have only DVD RAM Drive but i dont think it should matter. AS I checked the System Requirements of the game, there is no telling as to whatever DVD RAM or DVD ROM. I am pretty much sure that I had the Systems better from the minimum requirements particularly VGA,OS and everthing. The game runs only for the Multiplayer but when I play the singleplayer, the error starts to destruct my day. huhuhu help T_T T_T T_T