Online Casino
An online casino is available everywhere there is an internet connection. Even most mobile phones are accessable for casino gaming. If you find a casino that you like you can stay loyal to that casino and play only their casino games. Most casinos offer the same base of casino games. You might be able to find variations but mostly they are basically the same.
Online Casino Software
The difference is the type of software that they use and this is of course a matter of personal opinion. Some prefer software like Playtech or Microgaming some prefer others like Netgaming etc.
Casino Promotions
The real difference is the casino promotions the casino rooms offer. Some casinos offer very large signup bonuses and other kind of promotions. But conditions always apply and make sure you read these conditions thoroughly before signing up at an online casino. The bonus conditions can always be found at the casino promotion section at the casino website.