Roblox is a multiplayer game creation platform aimed at children to teens and allows people of all ages to create their own games and play games that are made by others. This ensures that there is always something new to play as new games are created daily. Roblox has been successful for a while and is one of the world's largest game creation platforms targeted at children and teens.
Roblox offers a variety of user-made games with tens of thousands of players even. There is almost no way to run out of games to play on Roblox and they are mostly high quality or fun to play. Children and teens will enjoy Roblox and adults might too.
Roblox is free, mostly. It doesn't cost anything to sign up for an account and you can play 99% of the games without paying anything. This forces the game into 'freemium' where certain aspects of the game require money to access or if you pay then you can get a certain item faster. Things like these are bought with 'robux' which is Roblox's virtual currency. As of now, you can buy 400 robux for $4.95. Also, there is a membership called BC (Builder's Club) and comes in three versions for varying lengths and prices.
Roblox is a game creation tool, meaning you can create games on the platform and publish them for others to play. Without paying, you could probably earn a little 'robux' from Roblox but if you want to earn more I suggest buying some 'robux' or BC to make use of the benefits of paying. Games in Roblox require building, scripting and graphics design. Learning how to utilise these three areas is key to a successful game.
http://wiki.roblox.com/index.php?title=Learn_Roblox is a good place to start.
Examples of games
Jailbreak has remained a very popular game on Roblox for quite some time, and I understand why. It's a cops vs robbers type of game which is a pretty simple concept: the cops try to capture the criminals. Some people hate/dislike Jailbreak because of its constant popularity. People play Jailbreak for many reasons such as the vehicles you can buy in the game or the gameplay of escaping jail.
Ever played Club Penguin? Meepcity is a roleplaying game aimed at young children. I don't really like this game but I don't hate it. Each player has their own 'Meep' and is tasked to look after it. The meep is a type of pet. The player can fish to earn money to spend on their house and decorations. Some people just focus on the roleplaying aspect more than others.
This game is an FPS game, you could compare it to Battlefield or Call of Duty. It is very popular on Roblox and I get why; it is very high quality. I believe that skins are a major part of this game's huge popularity which keeps players returning for more.