Someone who plays all games on the easiest modes. Couldn't care less to learn about the game and just button mash.

Gets insulted when someone teaches them all the moves they can use in the game.

Imo it's all about how you play games. Pretty much every game can be casual or hardcore depending on how you play.
You can sit down with Super Mario Bros. and just play through it for fun, which would be a more casual way of playing it. You can also try to get a high score, beat the game without dying, do speedruns etc. which would be a more hardcore way of playing. Generally speaking, of course.
Most gamers are a bit of both and extremely few are 100% casual or hardcore.

I'm also wholeheartly against the principle that hardcore gamers would be 'more' gamers than casual gamers, just because they prefer a different type of games. That's just something insecure people use to sound more 'elite'.

A wide variety of games? lol. 

I've had friends who played L4D, Counter Strike, Half-Life for months at a time. Does that mean they are casual? They are only playing one type of game. Same goes for friends that play an MMO for months on end... or Starcraft. Even though those games are considered "hardcore". Heck, when Viva Pinata came out I didn't play anything else for 2 months... 

My idea of a casual gamer is that of a person who doesn`t feel like investing a lot in gaming, be it in time, money or even deep experiences.

People who only plays one type of game and people who don't play certain type of games because they are afraid of what others think of him,

Someone who shuns games that require something to get into or simply someone who plays games seldom and in short intervals or both of the above.

Someone who doesn't take gaming seriously, in other words people who are misinformed or just don't know what they're talking about at all.

However, we can make fun of casual gamers but they are the ones that drive big sales numbers.

That's half this site :3 

Aww don't say that a good amount of people here seem to know what they're talking about.