Twitch has become an absolute giant these past few years, so it shouldn't come as much of a surprise to hear it will soon be not only a place where you can watch games, but also a place where you can buy them. And as an added bonus, any purchase made through a partnered channel will see 5% of the profits going to the streamer themselves.
If you're interested in seeing how this works in action and what the new UI is like, you'll find the recently posted trailer right below. But if you don't feel like watching an entire trailer about an online store the only somewhat important piece of information you need to keep in mind is that each purchase above $5 will earn you a Twitch Crate - a digital loot box that contains in-game emotes, chat badges, or some Bits. Anyway, here's the video:
The Twitch Store will be arriving in the spring of 2017, or in other words, pretty damn soon! All payments will initially be done in US dollars, though purchases can be made worldwide via the use of an Amazon Account. As for the games themselves, there's only going to be a select number of publishers and developers participating in this program, though if it ends up being a success I think its safe to assume that list will greatly expand.
If you would like to learn more, or just check out the official FAQ, you can find everything you're looking for over at the announcement page.