James Martin/CNET

TRACY, Calif. -- Amazon employee Rejinaldo Rosales used to wander stacks of shelves to pick up merchandise for orders before finally returning to his station to place them in bins and send them to their next stop.

But this summer, squat orange robots, called Kiva, began zooming around the shelves instead, picking up goods and carrying them to Rosales at his station. The result? What used to take hours of walking can happen in mere minutes instead.

Rosales, 34, who works at an Amazon fulfillment center in this Central Valley city about an hour and a half away from San Francisco, said he likes his new robotic coworkers. While walking the aisles was "good cardio," the new system lets him get through more orders since he stands in one place, he said.

"We don't socialize as much, but it's more efficient," Rosales said as the bots zipped around behind him on the eve of Cyber Monday, when Amazon showed off its latest generation of Kiva robots to a group of journalists.

Now playing: Watch this: Meet the robots making Amazon even faster

The fleet of machines -- installed in 10 of Amazon's US warehouses in California, Texas, New Jersey, Washington and Florida -- enable the company to deliver millions of items to customers. Along with many other retailers, the online shopping giant started its Black Friday sales a week early, building up to one of its busiest days of the year -- Cyber Monday. Last year, customers ordered more than 36.8 million items globally, or 426 items per second, according to Amazon.

Amazon expects that number to go up this year but wouldn't say by how much. ChannelAdvisor, an e-commerce company that services and advises retailers, reported that Amazon's holiday sales were already up by 24 percent on Black Friday and up 45 percent on Saturday.

Amazon isn't alone; online shopping is mainstream. Customers who used to line up at cashiers' stands are buying items from their couches instead. They're also using apps while in stores and mixing orders between the real world and the virtual one as they seek out the best deals and most convenient deliveries. Companies like Amazon have ridden this wave from $71 billion in sales 10 years ago to $4.5 trillion last year. Forrester expects this year's holiday sales to hit $89 billion, an increase of 13 percent from last year.

James Martin/CNET

Part of the way Amazon is keeping up are these Kiva robots. Originally made by a company Amazon bought in 2012, the devices have since been integrated with its warehouse technology. There are 15,000 Kiva robots spread across the 10 warehouses in the company's network, which has more than 50 facilities in the US.

"Robots are essential for meeting that kind of demand," said Ken Goldberg, a robotics professor at the University of California, Berkeley. Humans just can't work as fast.

Take me to your employer

As the Kiva robots speed up the pace of Amazon's warehouses, they raise a looming question that's being asked across the modern world: how much of what a human does can be done by a machine instead? The automotive manufacturing industry already knows the answer, as do various technology makers. Foxconn, one of the world's largest electronics manufacturers and a key supplier for Apple, was reported to be replacing workers with robots a couple years ago. And take a ride across San Francisco's famous Golden Gate Bridge and there won't be a toll taker in sight -- the process is entirely automated.

The trend doesn't appear to be stopping, and experts say robots will take over half of all human jobs within 10 to 20 years. A Pew Report in August said a vast majority of people it surveyed said they expect robots to "permeate wide segments of daily life by 2025, with huge implications for a range of industries such as health care, transport and logistics, customer service and home maintenance."

So far, Amazon said it hasn't eliminated any jobs with the introduction of Kiva. In fact, the company says it's hired more people in that time. Amazon wouldn't say how many jobs it's added after incorporating Kiva, but overall it's hired 61,110 employees since 2011, the year before it bought Kiva. That's roughly doubling its employee base over the past two years, though the company saw a decline in that growth last year.

"People play a crucial role in fulfillment for Amazon," said Dave Clark, Amazon's senior vice president of worldwide operations and customer service. "Take gift wrapping. Can we do it by machine? Yes. Does it look the same? Could you still have the same personal touch? No."

Still, Amazon is working on robots that can grasp items, bringing them one step closer to being able to replace the role of human pickers like Rosales.

In the meantime, Amazon encourages cooperation by writing some of its most enthusiastic employees' names on the Kiva robot's exterior shell. In Tracy, 87 robots are named after employees in the building, thanks to contests the company holds.

From a robot to your doorstep

The way Kiva robots work in Amazon's warehouses may seem limited, but the company says the work they do shaves more than an hour off an average order. At Amazon's fulfillment center in Tracy, people could be seen alongside various machines.

A product begins its life in an Amazon warehouse as employees unload packages from a truck onto a conveyer belt. A line of about 25 people stand near one conveyor belt, unloading boxes and cutting them open.

Employees then unpack the boxes and place them in carts that are brought to other workers who sort the items into the shelves.

The items appear to be shelved randomly, but they're actually organized based upon a computer algorithm. The result: one shelf in the Tracy warehouse had a My Little Pony toy, a roll of fluorescent tape and copies of Hamlet, smushed up next to one another.

James Martin/CNET

Tracy's 3,000 Kiva robots pick up shelves of products from the warehouse floor and bring them to a human employee who picks items and then packs them for shipping. This saves Amazon time and presumably money as well, though the company declined to say how much. Amazon's Clark said the average amount of time it takes to grab an item from a shelf and stuff it in a box is now about 15 minutes per order, down from an hour and a half. Amazon has steadily improved the robots since Kiva's acquisition, as well. The latest Kiva robot can move 50 percent more inventory out of the center than its predecessor could.

As Kiva's robots zoom around Amazon's warehouse floors, they avoid running into each other by using sensors that talk to one another. This technology makes Kiva one of the most advanced robotics systems being used today, according to Goldberg, the UC Berkeley robotics professor.

"It's more expensive than just having a bunch of static shelves, but it pays off if it has that much volume," Goldberg said.

So what happens if a robot fails? Amazon has human engineers, who can usually fix a robot within a couple hours. Amazon tries to makes sure no more than 10 robots per floor are out of commission at one time.

When they're working just right, they arrive at Rosales' station in a line. A computer screen above a set of conveyor belts tells him which item to grab and where on the shelf it's located. He grabs the items and throws them in a yellow bin on a conveyer belt to his right. Once he's done, he hits a button and the conveyor takes the bin away to be packaged and shipped.

He doesn't have a robot named after him yet, but he hopes that will change.

"I'm looking into it," he said. "That'd be cool, huh?"