This article will cover the two options you can add new Sales/Discounts to your website.
Shopping Cart Price Rules: These are These are discounts which get applied after the products have been added to the cart. Things like coupon code, free shipping, discounts in order total, buy X and get Y free, etc.
Catalog Price Rules: These are discounts that discounts that get applied to an entire section or category before being added to the cart. For instance if you want a percentage off a whole category.
Shopping Cart Price Rules