Cosplayer is a short way to say Costume Player. These are people who dress up as comic book characters, cartoon characters and movie characters and replicate their costumes to be as close as possible to the character they are portraying. We called them " nerds " when I was young along with other terminology not fit for polite society. 
Hardcore Halloweeners then?
No. We do this Year round, and we take accuracy in our costuming far more seriously than Halloween costumes. There are people that spends thousands of dollars on costumes. Cosplayers range from the Trekkies you see in costume To Troopers the 501s "Vaders Fist" Legion and There are of course the Various types of Furry and Scalies Oh and Some Bronies. A very specific subset of Cosplay are the Various Military ReEnactors. I Myself Portray several characters, Mario, Wario, I've also portrayed Gluttony From Full Metal Alchemist, And several other characters of... shall we say mass.