From Aladdin to X-Men, the site covers a variety of costumes seen in movies, tv shows, and video games. Each entry gives you a basic rundown of the pieces and provides links to where you can purchase them. There's even a form for you to suggest future costume overviews.
My guess is that the creators, Jad and Nate, have probably set up affiliate links with a lot of the vendors which is a great way for them to cover the costs of hosting this free resource. So if you do end up using their guides, purchasing through their site link is a cool way to say thanks.