Jack Dorsey's Square Reader has revolutionized the way small businesses operate. By plugging the small device into any Android, iPhone, or iPad, the user can run credit and debit cards instead of requiring cash. The funds deposit directly into the businessperson's bank account. Square has two pricing plans for its product: It either takes $275 per month or 2.75% per swipe. For customers, the Square Wallet makes purchasing easy when they visit businesses that use Square Register. They simply download the app, check in to the place they're patronizing, and say their name at checkout to pay.

Birchbox

Co-CEOs: Hayley Barna & Katia Beauchamp

40 Under 40 rank: 31

Feeding a beauty product addiction is easier than ever with Birchbox. The brainchild of co-founders Hayley Barna and Katia Beauchamp -- who met at Harvard Business School -- and their editorial director Mollie Chen, the company sends a box full of product samples to monthly subscribers. The company caters to both men and women and has expanded to France, the U.K., and Spain.

Glow

Co-Founder: Max Levchin

40 Under 40 rank: 28

Science, family planning, and colorful calendars collide in Glow, an app for couples who are trying to get pregnant. With Glow's to do list feature, ability to track a woman's monthly cycle and ability to input variables such as mood, pregnancy test results, and consistency of cervical mucus, the app becomes a well-rounded diagnosis of health and fertility. The company also features the option to donate to a pool of funds that can help couples pay for reproductive treatments after a 10 month period.

Uber

CEO: Travis Kalanick

40 Under 40 rank: 13

Outsourcing at its finest: Uber's app blends the convenience of online ordering with the luxury of having a personal driver. With the Uber app, users can order a driver-for-hire to taxi them from point A to point B. Riders who sign up pre-enter their payment information, so the ride itself is cashless and easy to split between other users. Plus, riders can track their driver's progress via GPS on their smart phone, ensuring a reliable pickup. Vehicles come in a variety of styles, from economy to mid-range sedans to SUVs to upscale rides. Uber is now available in 18 countries and 24 U.S. cities, and it's growing.

Fiat

Chairman: John Elkann

40 Under 40 rank: 4

The Fiat 500 has been the surprise hit of the past year. The Italian subcompact has made a number of top lists: Kelley Blue Book's 10 Coolest New Cars under $18,000, Consumer Digest's Best Buy/Subcompact Cars, and a Top Safety Pick from the Insurance Institute for Highway Safety. Its charming "Italian Invasion" and "Backseat Italians" commercials have further branded the 500 as a fun, sexy, and functional automobile.

SolarCity

CEO: Lyndon Rive

40 Under 40 rank: 23

SolarCity wants its customers to go clean and save green. The company starts with a free consult to a home or business interested in solar power. If a consumer decides it's a go, then SolarCity takes care of everything from designing a setup to permitting to installation to maintenance. Over time, the company promises, a customer's utility bills will be much lower compared to other energy sources -- while making nice with Mother Earth.

Ralph & Russo

Creative Director & CEO: Tamara Ralph & Michael Russo

40 Under 40 rank: 39

The painstakingly detailed fashions of Ralph & Russo reveal an ever-elusive quality that separates great designers from the mundane. The brand has dressed Angelina Jolie and Beyoncé on the red carpet, as well as royalty including Princess Ameerah Al Taweel of Saudia Arabia and Sheikha Mozah Bint Nasser Al Missned, Queen of Qatar. Tamara Ralph and Michael Russo founded the company in 2007 in London and have since fostered an air of exclusivity: Even browsing their website is by invitation only.