From an outside perspective, Halloween costumes and cosplays look exactly the same. But from an inside perspective, they are quite different, and I thought the best way to explain the differences was to ask cosplayers themselves.
For those who may not know, Cosplay stands for costume play and originated in Japan. It is the practice of dressing up as a character from a movie, book, or video game, and can often be seen at conventions and meetups. Yes, it is wearing a costume, but there is so much more to it, including the following.

Lifestyle
“I’ve come to learn that while Cosplay is fun, it’s more of a lifestyle than anything. Being a Cosplayer, you’re always thinking of what’s next, even while you’re currently working on another costume. With Halloween, you may start thinking about it earlier rather than later, but it’s more of a casual “What should I be for Halloween?” For Cosplay I’m researching every little detail of the costume, different fabric types, and how can I make sure I’m not spending too much money.” Jessica Dalton of Two Married Geeks

Intent
“The biggest difference between cosplay and Halloween is the intent. Cosplay is not only about the clothes you’re wearing, but the character you’re portraying. When in a cosplay state of mind, you embody the character. For some it’s a great way to escape everyday life. I think it’s closer to a theatrical performance. When you’re wearing a Halloween costume, you’re not trying to be anyone but yourself, with the exception of things like haunted houses.” Jennifer Pawley of The Geek Forge

Community
“Sure many people are dressed up on Halloween at one time, but there isn’t that sense of belonging that happens when you’re at a convention cosplaying with thousands of other people. On Halloween, you might get a couple of “cool costumes” but at a convention, you run into people chasing you down just to get a picture of your carefully crafted Cosplay.” Jessica Dalton of Two Married Geeks

Time
“You can buy a full Halloween costume or have fun making one only a few days before wearing it and it will be cool. As far as cosplay goes, it can take months to put every single handmade detail together, always struggling to reflect perfectly the character it’s based on. You will not only dress as someone (or something ^^), you will literally impersonate, get into character; that includes behavior, mimics and speech, it has to be as real as possible – all the contrary to dressing up. You will wear it proudly and will always be happy to do some photoshoots to immortalise your work. Some even can become professional cosplayers whereas a professional Halloween costumer is less likely to be acknowledged and successful, the latter being limited to only one day a year.” Nyanla La Geekette of Geekeries de Nyanla
Make sure you check out the sites of the awesome ladies who shared their insights and experiences of cosplay!
An interesting and excellent article, but there people such as myself consider themselves costumers long before cosplay. We don’t become our characters (unless we’re in fan group) but we put a lot of time into our costumes, even when adapting mundane articles of clothing. I wonder how we fit in the costume vs. cosplay discussion. My personal experience with cosplayers have been mixed, though mostly positive.
I didn’t realize how much preparation went into Cosplay. I’ve seen many people dressed in their character but never knew they “became” them.
I don’t go to conventions or participate in cosplay, so for me, Halloween is a safe opportunity to dip my toes into the cosplay waters – my Halloween costumes end up being way more elaborate and detailed than my friends’ (down to having the character’s exact lipstick and nail polish this year). It dawned on me this weekend that I was the only one who really did my hair and made sure my (1940’s) shoes were appropriate for my costume, rather than just putting on a costume and maybe some face paint, but I love it!