Jack Dorsey: People who are using it to sell things on Craigslist to holding garage sales - campaigns -- the Obama campaign and the Romney campaign both used Square to raise funds.

Dorsey says merchants like Square because the fee is less than some credit cards charge. And business at Square is booming, accelerating in three years from zero to $12 billion in annual transactions.

Jack Dorsey started thinking about the software he's built in Silicon Valley nearly 30 years ago when he was growing up in St. Louis. He had a speech impediment as a kid and spent a lot of time alone at home, playing with computers when he was eight. He taught himself how to build computer programs before he was a teenager. Dorsey was fascinated by trains and maps and he used to spend hours down at these train yards.

Lara Logan: Most kids would have pictures of football players and girls on their walls or their favorite bands.

Jack Dorsey: Right.

Lara Logan: And you've got maps.

Jack Dorsey: I have maps.

Lara Logan: And trains.

Jack Dorsey: Yeah.

Studying trains was the beginning of his life-long obsession to learn how things work in the real world and translating that into the virtual world. Young Jack was intrigued by the messages he heard coming out of the St. Louis emergency dispatch center. At home he listened to it all on a police scanner. And he was struck by the fact that everyone talked in short bursts of sound - a system of communication that later inspired him to invent Twitter.

Jack Dorsey: They're always talking about where they're going, what they're doing and where they currently are. And that is where the idea for Twitter came was now we all have these cell phones. We had text messaging. And suddenly we could update where I was, what I'm doing, where I'm going, how I feel. And then it would go out to the entire world.

As a teenager, he created software that tracked the movement of emergency vehicles on a map. Then he tried to get a job with a large dispatch company in New York but there was no contact information on their website.

Jack Dorsey: I found a way into the website. I found a hole. I found a security hole.

Lara Logan: Is that-- are you-- is that the same thing as hacking?

Jack Dorsey: It's-- ha-- yes. Hacking-- hacking is-- hacking is-- is--

Lara Logan: A crime.

Jack Dorsey: Well, no. Criminal hacking is a crime. Hacking is actually a--

Lara Logan: Hacking for a job application is not a crime?

Jack Dorsey: No, no, no, no, no. No, not a crime at all. And I emailed them and I said, "You have a security hole. Here's how to fix it. And I write dispatch software." And--

Lara Logan: And they hired you.