It features a magnesium case and built-in stand, said the company. A keyboard and touch pad are built into its cover, and cameras are built into the front and back.
It will come in two versions, the company said, each weighing less than two pounds.
"It's a whole new family of computing devices from Microsoft," said Steve Ballmer, Microsoft's CEO, from the stage of a Los Angeles auditorium packed with reporters. "It is a tool to surface your passions and creativity."
Some critical specifications were not mentioned -- including price, screen resolution, battery life or the specific release date.
Windows 8 was previously promised for consumers for later this year.
The Surface is clearly meant to compete with Apple's iPad and other tablets already on the market, and Microsoft demonstrated several features existing tablets only have as accessories, if at all.
The basic version, which weighs 1.5 pounds and is only 9 millimeters thick, is designed to run on a low-power version of Windows 8, called RT. The two-pound Surface Pro will use more powerful Ivy Bridge processors from Intel. The Pro, 13.5 mm thick, allows users to type on its keyboard, move things with their fingers on the touch-pad screen, or write on it with a stylus.
The basic version will be available with 32 or 64 GB of memory. The more powerful Pro will be configured with 64 or 128 GB.
Panos Panay, who led the design of the Surface, said it was put together so that "the hardware fades into the background." He said the tablet is meant to work seamlessly with Windows 8, so that users can focus on what they're doing with it instead of fussing with equipment.
"When you touch it, you are going to want to hold it," said Panay.
He talked about its exacting specifications. The magnesium exterior, he said, is no thicker than a credit card or a hotel room key.
"The Surface is a PC. It is a tablet," Ballmer said.
The new devices are an unusual -- and potentially risky -- foray by Microsoft away from its core business in computer software. Aside from its Xbox gaming system, it has had few successes in marketing its own hardware.
Microsoft made its name -- becoming a dominant force in personal computing -- with such software as Windows and its Microsoft Office programs. Windows 8 is the first version to be designed, not just for computers, but for tablets and other mobile devices with touch screens.
The company kept the subject of its announcement a secret up until the very last moment. It invited reporters to the event with the most enigmatic of emails, saying only it would reveal something "major." Technology blogs went wild, some suggesting Microsoft would do something in collaboration with Barnes & Noble, but that was one of the few rumors actively quashed.
Microsoft has not had a competitor for the iPad since Apple released the original version in 2010. Windows 8, which is to be available to consumers before the end of the year, has been the company's hope to compete against Apple's dominance in the tablet market. Apple sold 3 million of its new iPads in the first three days after its launch.
Microsoft currently sells its own computer accessories, including mice and keyboards, but hasn't had much success making its own phones or mobile devices. The Zune, Microsoft's iPod competitor, was recently discontinued and the KIN line of phones, which was aimed at teens, was pulled from the market within months of release.
Ballmer said the company took its time with the tablets, working quietly on its new machines to work out all the details.
"We took the time to get Windows 8 and Surface right," he said.