Yeah same as D&D. Complexity varies from game to game. In some games, for instance, you just have hit points, and once you lose them all you're dead. It gets more complex when you have hit locations - you can get wounded in an arm or leg which affects your attack strength or speed until it's healed. And then it can depend on how many different types of dice you use. D&D uses 4-sided, 6-sided, 8-sided, 10-sided, 12-sided and 20-sided dice. But the old (and very successful) Star Wars RPG only used 6-sided dice.
The different systems are endless. I read the Aliens RPG a couple of times and it relies on a lot of detailed referencing of tables. It can make the game realistic, but extremely slow.