Collision Damage Waiver (CDW) and Loss Damage Waiver (LDW) are both basic vehicle protections that limit the driver’s liability in the event of accidental damage, theft, fire and loss of use. The main difference between them is that CDW comes with an excess while LDW doesn't.
It is highly recommended to hire one of them. These protections can be mandatory to some rental companies while others might prefer to block a higher amount for the security deposit on your credit card.