Liquidated damage/Ascertained damage: 
In the event of the contractor not completing the work in the scheduled time (or also within the extended time, if allowed by the authorities on valid grounds) due charge is levied on him. Such sum is usually a specified sum per day for each day of delay - this sum is termed ascertained or liquidated damages, and not as a penalty. It is a fixed stipulated sum of penalty payable by the contractor for the damage having no relation with the actual damage. 
Unliquidated damage: 
It is a fixed stipulated sum of penalty payable by the contractor for the ordinary damage having relation with the actual damage. 
Penalty: 
The penalty is a sort of fine for non-fulfillment of the terms of the 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 the delay in completion, for bad work, etc.