I recommend you go to Gamasutra.com and GameDev.net and check their game design sections, you'll find a lot of articles on the topic and will learn that it's "the art of meaningful interactions".
It's about game balance and gameplay, at UBISoft they consider gamedesign like a music partition, in which different notes would be different buttons/keys...
BTW, IMO anyone having an internet connection should first try to find infos on a given topic with his/her favortie search engine before asking.
(No offence intended.)