I am interested in using one domain for multiple sites. I am not sure how to make these sub-category sites. For example say my domain name is "REVIEWS.COM" and say I want to have a review on golf equipment, so I want GOLF.REVIEWS.COM to take people to the golf equipment reviews. For a normal site I obviously place an index.html file in the http folder. What do I do different to make this work? I could probably research this myself if I knew the correct terminology for what Im trying to say.

That depends from your webhost. You may need to contact them to set up a subdomain for you. You could also try to browse your administration panel for something like "add subdomain".

Once it is created, it will appear as a new folder and it will have the name you chose. Simply put your html files in it and it should work the way you want : subdomain.yourdomain.com

If you try and simply create a folder it won't work as a subdomain and you will have to access it www.yourdomain.com/subdomain

Definitely something to do with your hosting. Fairly easy to do with C-Panel if you have that with your hosting.

Thanks , I found what you are talking about when logging into my server.