Cosplay, also known as Costume Play which literally no one will ever call it by, is the act of dressing up like ones favorite movie, television, comic book, anime, or video game character. It isn't Halloween though, where one simply buys some cheap costume knock off on the shelf and calls themselves, say a Thor. No, it's someone going in with their own two hands and designing something befitting of their love of whatever fandom, they are a part of and enjoy.
This is where geekdom reigns supreme. It's for all those people in theater or in engineering, or who are in make-up and hair, or into effects, or sewing, or just overall creative types who want to pay homage to the characters they know and love in the best way. Many people put up to a years worth of work plus into some of the costumes and designs they come up with, often having to manufacture or re-create the items from scratch because sometimes there is no sewing pattern or blueprint of the costumes available to the public. It can be a true, if not expensive, labor of love from the fan, but they don't do it for money (unless they are going after a competition prize), but rather for all the other fans out there just like them that love these comics and movies as much as they do.
It doesn't just stop though at just the costume. For a lot of Cosplayers, the act of dressing up isn't just about just being a fan, but also for many there is a deeper message of acceptance running through the likes of Comic Cons around the world. So many Cosplayers were or are treated in their own lives as less than, or bullied, or made fun of on a constant basis for liking what they like or being who they are. Cosplay was and is a form of escapism where they can embody the elements of their favorite superheroes or television characters with other like minded people who get it and who understand this world and what they are going through. Everyone needs a community like that where they feel safe, where there is an atmosphere of love and fun and where they can go to when things in their life get bad.
Anyone can be a part of the Cosplay world whether you make up a simple costume to pay homage to your favorite character or collaborate on something really instensely spectacular. It's about a love for the work many many professionals from writers, to producers, to the artists across the world, who first decided to bring these characters to life.