Tech companies say virtual reality is about to change the world. Will it live up to the hype? Here's everything you need to know:

What is virtual reality?
It's a technology that transports users into three-dimensional, 360-degree virtual worlds that feel "real.'' By donning a headset, you are immersed in a rich, stereoscopic, and responsive environment that tricks the visual cortex into believing what it sees. Turn your head, and your view shifts. A number of VR headsets are expected to go on sale to the public in a matter of months. The most hyped is Facebook's Oculus Rift. But at least three other tech giants — Google, Sony, and Samsung — are pouring billions of dollars into their own ambitious virtual-reality projects. In the near future, users could put on one of these headsets and find themselves lying on the sands of an exotic beach, standing on the rim of the Grand Canyon, or chasing bad guys through the streets of virtual cities. "This is the first time that we've succeeded in stimulating parts of the human visual system directly," says virtual reality engineer Michael Abrash. "I don't get vertigo when I watch a video of the Grand Canyon on TV, but I do when I stand on a ledge in VR."

Hasn't this technology been tried before?
Yes. Virtual reality enjoyed a brief heyday in the 1990s, but the headsets proved too bulky and the graphics too crude and clunky. Motion sickness was also a major problem. The onscreen graphics couldn't keep pace with VR users' head movements, and that tiny but perceptible delay — known as latency — created a powerful nausea that turned people off. But thanks to massive progress in mobile computing, headsets are now lightweight and latency is down to just 20 milliseconds. That means no more motion sickness, plus a more immersive experience. "I describe [it] as visceral," said British psychologist Dr. Ashley Conway, after using a VR headset to ride a virtual roller coaster. "You feel your stomach lurch, even though there is a part of your brain telling you that this is an illusion."

How will we use virtual reality?
Proponents of the technology say some workers will no longer need to commute to a physical office and could instead pop on a headset and telecommute into a virtual meeting with colleagues. At home, people could use VR headsets to scroll through their social media feeds and jump right into a friend's video — and feel as if they were actually there. Microsoft is working on a connected technology, known as augmented reality, that superimposes high-definition holograms on real-life rooms. Wearing one of the company's HoloLens headsets, you could soon sit next to a virtual puppy on your real-life living room sofa while watching a movie on your hologram big-screen TV. But VR technology isn't just for entertainment; it's already being put to more serious uses.

Such as?
Psychologists are currently testing VR headsets on patients with severe phobias and social anxieties. Patients with a crippling fear of heights, for example, can practice standing at the top of a virtual skyscraper without ever leaving the doctor's office; through VR, veterans with PTSD can revisit a traumatic wartime event, a process that experts say can aid recovery. Surgeons use virtual reality to practice rare and difficult procedures before performing them on actual patients, while NGOs have produced VR fundraising documentaries that allow potential donors to walk around Nepal's capital, Kathmandu, after the recent devastating earthquake, or experience life inside a Syrian refugee camp. The technology, says filmmaker Chris Milk, could become a powerful tool for charities. "There's something about sitting on the same ground someone else is sitting on," he says, "that changes the way your brain registers humanity."

Could VR make us even more screen-obsessed?
That's a genuine concern. Several World of Warcraft gamers have committed suicide or collapsed in recent years after playing the popular video game for 20 hours straight or longer; such incidents could become more common if VR allows gamers to effectively live inside an imaginary, interactive land. Virtual reality could even result in non-gamers shutting themselves off from the outside world, warns Stanford University psychiatrist Dr. Elias Aboujaoude. "To some degree, this has already happened with the internet," Aboujaoude told The Atlantic. "We may stop 'needing' or craving real social interactions [because] we are so immersed in virtual life."

Is that likely to occur?
In the long run, perhaps, but not at this stage. Virtual reality still needs a lot of work before it becomes truly — and perhaps dangerously — immersive. The next generation of VR headsets won't engage senses other than sight and hearing, so users won't feel the heat of the sun while they walk along a virtual beach, or smell the sea air. And for the next few years, VR headsets will only work when hooked up to expensive gaming PCs or consoles, limiting the number of potential buyers, at least for now. But in our hectic and often stressful lives, the pure escapism of virtual reality could ensure the technology's success. After a hard day at work, you could relax by putting on a headset and immersing yourself in a favorite vacation video. "Who wouldn't want to do that?" asks Chris Dixon of Andreessen Horowitz, a venture-capital firm that has invested in Oculus Rift. "In some ways, the biggest competitor to virtual reality might be a bottle of wine."

Virtual porn: The rise of 'teledildonics'
The pornography industry has never been shy about embracing new technology. Porn producers were among the first to see the commercial potential of VCRs, DVDs, and web videos, and the industry is now experimenting with virtual reality. Adult VR content is forecast to be a $1 billion business by 2025, according to investment bank Piper Jaffray, and the third-biggest virtual-reality sector after video games and NFL-related content. A French adult-film company has already shot a 3-D, 360-degree pornographic movie for VR headsets; another European firm has developed technology that re-creates the sensation of an adult-movie actress whispering into the ear of the headset wearer. A particularly creative field is "teledildonics": internet-connected sex toys that simulate what the performers are doing on screen. The possibilities of virtual porn are endless, says British futurologist Ian Pearson, who predicts that by 2030 most people will have engaged in some form of VR sex. You'll be able to "play with an artificial intelligence character you've designed to fulfill your wildest fantasy," says Pearson, "and nobody but you need ever know."