Roblox is an online social game that allows you to create your own avatar, customize it however you want, and then interact within a virtual 3D world with other players. The best part about it is that you can play with your friends no matter where they're located, as long as you're connected to the same network. That said, Roblox does require a decent Internet connection. Otherwise, you'll get a 'network error' message.
The way Roblox works is very similar to the classic game Minecraft (although this is definitely not a copy of Minecraft). Basically, you use a sort of 'virtual block-building' system to create anything you can imagine. You can even buy and sell online within Roblox.
One of the most interesting features in Roblox is that it's fully customizable. You can change the skin of any of the characters you meet, add sound effects to the movements of your hands when you 'play' with the virtual blocks, etc. Basically, you can enjoy a fully immersive gaming experience without ever having to 'quit' the game to go pick up something off the kitchen table.
You can have your groceries brought to your door by a Roomba if you want..Roblox is an excellent social game that has millions of users around the world.