Adding onto charank, the only similarity to Minecraft that Roblox has is its blocky characters. The developers of Roblox even tried to implement more realistic models for players to play as, but that was received poorly.
On Roblox, you can create just about anything that your heart desires.
Similarly, Minecraft mods can be used to create whatever you want.
If you are a player in singleplayer mode, Minecraft offers complete control over the game elements from the start. But a Roblox player can't simply terraform a server map when they wish; nor grant themselves everything in the game using creative mode. But, you can do that when your developing a game.