Samsung first talked about Samsung Pay when the Galaxy S6 was announced, but the service wasn't actually ready. Now it's all systems go for a September launch in the US. Samsung Pay will allow you to make wireless payments at almost any payment terminal that supports regular credit cards, and it's only compatible with Samsung's newer premium phones.
Samsung Pay uses both NFC and MST (magnetic secure transmission). The former is standard on most phones (looking at you OnePlus), but MST is exclusive to Samsung right now (including the Galaxy S6, S6 Edge, Note 5, and S6 Edge+). The phone basically beams magnetic stripe data at the card reader from a few inches away. It should support almost any credit card, including many store cards.
Samsung Pay is secured by KNOX and can be used with fingerprint verification. That means if you've done something to trip KNOX on your device, it's unlikely Samsung Pay will work. South Korea will get Samsung Pay a little earlier on August 20th, but a limited beta test in the US starts on August 25th. Samsung also plans to roll Pay out to the UK, Spain, and China eventually.