Well actually, there are many similarities as well as many differences.

As far as the games go, they are very similar and use the same winning odds as land-based casinos. Online casinos are equipped with a very advanced RNG system (Random Number Generator) which insures that all casino games are legit and fair and offer a decent amount of winning odds. This system is tested every now and then by an independent company called TST (Technical Systems Testing) which makes sure the RNG system is legit. Failure to prove so causes the casino to shut down immediately.

The ambiance is probably what is the most different from a land-based casino. You do not have the noises of the crowd around you, security guards walking around, interaction with other players, and so on. It is only you and your computer screen. However, online casino softwares try their best to recreate this ambience to give the player the feeling as if he is in a land-based casino.

So above are the major similarities and differences. It is obvious that each one have their own pros and cons and it all depends of the actual player. Some players rather play from their living room in pyjamas, while others like to get all dressed up and sit at the roulette table and interact with other players and having a good laugh (or a good cry). It all depends of who you are, what kind of player you are, and what ambiance you like.