Almost every credit card issuer has an online shopping portal. And this is how it works. They form partnerships with various online merchants. Let's use Apple store as an example. You go go to apple.com directly and order stuff online. Or if you have a Discover More Card, you could login into your account and go the shopdiscover section on the site. You then search for the Apple store and enter the site (it's the same site except that by doing it this way, your purchases are tracked). You then browse the store, select the products you want and use this card to pay for it. By doing so this way, you will earn 5% cash back (in the case of shopping with Apple, it varies with different merchants). In contrast, if you went to Apple store directly, you would not have received any cash back at all.
Earlier on, we made a bold statement saying we thought Discover had the best portal. While they do not have as many merchant partners as other issuers, they pay on average a higher rebate. The minimum starts at 5% and tops out at 20%. If we use Apple as an example, they are also partners with other issuers like Chase, Capital One etc. But with those portals, you only earn 2% or 3% rebates.
When this portal first started, there were less than 100 merchant partners. Now, there are more than 200. So once in a while, I do have to update the screen shots below!