Subcategories / subdirectories are fine. Examples:
yoursite.com/bingo/
yoursite.com/poker/
yoursite.com/slots/
Subdomains are like this:
bingo.yoursite.com
poker.yoursite.com
slots.yoursite.com
I read an article a while ago about subdomains, and the article said that Google treated each subdomain as a unique website. However, since each of these “unique” websites will have the same IP address, you should be careful if you link the subdomains together. From what I understand, sites can be penalized for excessive interlinking. I don’t know what is considered “excessive,” though.
Personally, I think it’s easier to use subdirectories. Just create a new folder in the main directory and you’re done. Subdomains require more work to set up, and I don’t know if there are any real benefits.
Most of the large casino portals I’ve seen use subdirectories, not subdomains.