Cart Discounts apply to your client's shopping cart balance. Dollar and percent discounts can be tiered based on the cart total, or they can apply free shipping to a cart. For example, "Get 30% off all orders over $500" or "Receive $5 off when you place an order by November 1st", or even just "Free Shipping on orders over $100".
Cart Discounts can be Automatic, or Promo Code based. Automatic discounts apply to all eligible items in a cart automatically, while Promo Code discounts require that the client enter a Promo Code in the cart or at checkout in order to receive the discount.