Any video game that you do not like. If you like the game, then it is a hardcore game.
Some insecure gamers will deride games they don't like as "casual games". The term has little meaning, except as an epithet hurled by who place a great deal of their self-worth in their videogames. A more mature person will admit that other people legitimately may have different tastes.
Casual games are any kind of game that is over hyped and over rated or just the exactly same thing as a previous version that was over hyped and over rated, these games are known by gamers as "crap" because even with all the perfect scores the games still have mediocre graphics and shitty plots that casual gamers think are good. Usually the only thing that makes a casual game not-total shit is the multiplayer, otherwise these games would get ratings lower than dirt.
With shitty graphics and a generally horrible campaign mode, the halo series is the indisputable king of casual games.