[Haskell] Monad and Monad transformer???

[Duong Thai Ha]

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