It is a fixed stipulated sum of penalty payable by the contractor for the ordinary damage having relation with the actual damage.
Liquidated damage:
It is a fixed stipulated sum of penalty payable by the contractor for the damage having no relation with the actual damage.
Penalty:
Penalty is sort of fine for non-fulfilment of terms of contract.
Every contract usually should contain certain provisions for the penalty for the breach of terms and conditions of contract as for not maintaining the progress, for delay in completion, for bad work, etc.