[Haskell] Monad and Monad transformer???

hi I am a newbee to haskell so please forgive me if the question is not appropreate. I am trying to get the idea of using Monad and have to understand the following code. Can you please tell me is how different Monad and Monad transformer is? And what is mean by

newtype State s a = State { runState :: s -> (a, s) }

--------------------------------- Define (State s) as a State monad

instance Monad (State s) where return a = State $ \s -> (a,s) m >>= k = State $ \s -> let (a,s') = runState m s in runState (k a) s'

Thanks lot for your help I really do appreciate that Best regard