A cosplayer is a person that is a fan of anime, manga, comics, gaming, certain movies or TV shows who dresses in a costume of a particular character and takes on aspects or traits of that character. COSPLAY comes from COStume and PLAY -as in PLAY acting. I hope that helps!!
Cosplaying is an expression of fandom. You are probably aware of fandom in a sense that fans of say, NASCAR, will often support their favorite driver by wearing T-shirts, hats etc. That is an expression of fandom.