An article coming from Renegade Kid, Jools Watsham blog.

For the purpose of this article:

Serious Gamer = A person who considers gaming their main source of entertainment.
Casual Gamer = A person who considers gaming just one of many sources of entertainment.
Something that I find interesting is people worrying about devices such as iPhones and iPads taking potential customers away from the 3DS or Vita. Who’s to say they were potential customers? Just because someone purchases an iPad does this mean it is a foregone conclusion that they would have bought a dedicated handheld gaming device such as a 3DS or Vita if the iPad did not exist? I don’t think so.
Back in the days of the NES, Master System, SNES, and Genesis/Mega Drive the majority of the gaming audience were serious gamers. The minority of the audience were casual gamers. I think it goes without challenge to say that period of gaming was successful in terms of money being made by console manufacturers and publishers. Business was booming.
Fast forward to today. There are millions of new players who are considered the gaming audience. The serious gamer is now the minority. The casual gamer is now the majority. There are more serious gamers than ever before, but there are a larger number of casual gamers now. Much more money is being made due to the larger audience. But, business is kind of a mess. It certainly is not booming; at least, not for everyone.
Console manufacturers and video-game publishers who can capture the imagination of the casual gamers win. Just look at the success of games like Nintendogs, Farmville, and the recent Call of Duty titles. I know. Some of you will think I’m insane for lumping Call of Duty into the same category as Nintendogs and Farmville, but hear me out.
I believe (and you’re welcome to disagree) that the majority of today’s Call of Duty players are dedicated casual gamers. Yeah, I know that sounds counter-intuitive. What I mean by this is that the dedicated casual players of Call of Duty devote their time to one game series. Sure, they might dabble in other titles while they’re gracing us with their presence in the gaming scene, but their main source of gaming entertainment is Call of Duty. They might in fact spend countless hours playing just Call of Duty, pushing other previous sources of entertainment to the side. But, when the day comes that they no longer wish to play Call of Duty, their interest in video-games is put back on an equal footing with the many other sources of entertainment. Whereas a serious gamer would naturally move onto the next game.