You've never met a 21-year-old like John Collison. At least not one this ambitious. Sober, direct, and brilliant, John and his 23-year-old brother, Patrick, have set out to do nothing less than disrupt the online payments industry and even go head-to-head with PayPal.

He prefers to describe what his 22-person San Francisco start-up is doing as "behavior changing."

The Irish brothers came to the U.S. to pursue their shared interests in math, science, and programming. Patrick went to MIT as a math major and John planned to study physics at Harvard. Then they both dropped out.

They had an idea, and it was gargantuan.

The problem that needed solving

If you own a small business and want to take credit card payments through your website you either have to get a merchant account—a cumbersome process that can take weeks to complete—or you set up a PayPal or Google Checkout account.

The latter option is what most businesses choose but it has a downside, too: Why give all that control to a third party and lose the opportunity to manage your brand and customer data all the way through the transaction?

Either way, the process of getting set up to take credit cards online is not an easy or short process. And that's exactly the problem Stripe wants to solve.

John will not divulge how Stripe's technology works, but by embedding some code into your website, he says you can be up and running to accept credit card payments online in a matter of hours, not weeks. Stripe charges a flat rate of 2.9% plus 30 cents per transaction.

"Everything is done online," he says. "We're getting rid of an awful lot of manual processes. We do a number of similar things to what the merchant account providers do but we do them instantaneously. And we do things like identity verification instantaneously."

Can Stripe take on the big guns?

Several interesting players are keen on Stripe's technology. In fact, PayPal co-founders Elon Musk, Peter Thiel, and Max Levchin all have invested in the company along with Sequoia Capital, which has valued the company at $100 million.

And users love it. Here's what entrepreneur Mark Bao wrote in a thread on Hacker News:

"Merchant accounts are a serious drag. I've opened a few and they've been nothing but headaches (especially if you're young--nobody trusts you.) Couple that with getting a gateway account, dealing with credit checks, monthly fees, monthly minimums, slow people in the payments industry, PCI compliance. Stripe takes payments and puts them behind a simple API. No crappiness and 1099 rules of PayPal. No more reconsidering the meaning of life like back when I had a merchant account."

Doubtful that this little start-up can shake up the likes of PayPal and Google Checkout? Last month Stripe took the curtains off a gallery listing some of the thousands of sites that have started using Stripe in the past few months, including the New York's Museum of Modern Art and Web publishing company Sugar, Inc. The start-up launched in September and John says for the last six months its customer base has grown 45% month over month.

And the big guys are taking notice. According to Inc.'s sister publication Fast Company, a source at Google Checkout says it's keeping close tabs on the Collisons.

To be sure, Stripe is one to watch.