Computing is changing. The news last week showed that loud and clear, as Microsoft bet big on Skype’s voice and video technology and Google announced partnerships with Samsung and Acer to build laptops running its Chrome operating system. These developments point to a future where computing form factors, interfaces, and operating systems diversify beyond even what we have today. The “Post-PC Era” is underway, but its definition is not self-evident.

First, some history. “Post-PC” has been a buzzword in the past few months, since Steve Jobs announced at the iPad 2 launch event that Apple now gets a majority of its revenue from “post-PC devices,” including the iPod, iPhone, and iPad—a major milestone for a company that was originally named “Apple Computer.” The phrase was also part of the public discourse in 2004, when IBM sold its PC unit and former Sun Microsystems CEO Jonathan Schwartz told The New York Timesthat “We've been in the post-PC era for four years now,” noting that wireless mobile handset sales had already far surpassed PC sales around the world. In fact, the “post-PC” concept is more than a decade old: In 1999, MIT research scientist David Clark gave a talk called “The Post PC Internet,” describing a future point at which objects like wristwatches and eyeglasses would be Internet-connected computing devices.

So what does “post-PC” mean, anyway? It doesn’t mean that the PC is dead: Forrester Research forecasts that even in the US, a mature market, consumer laptop sales will grow at a CAGR of 8% between 2010 and 2015, and desktop sales will decline only slightly. Even in 2015, when 82 million US consumers will own a tablet, more US consumers will own laptops (140 million). But, as Forrester explains in a new report out today, it does mean that computing is shifting from:

• Stationary to ubiquitous. Contrast the experience of computing on a desktop PC, in one place with a clear start and finish time, to that of the anytime/anywhere computing done on a smartphone or tablet. Ubiquitous computing is also more context-aware computing, aided by sensors like accelerometers, gyroscopes, and geolocators in smartphones and tablets.
• Formal to casual. In contrast to PC interactions with a formal start and finish time marked by booting up and shutting down, instant-on/always-on computing on smartphones and tablets fills in-between moments like standing in line or watching TV. 
• Arms-length to intimate. With desktops, computing is literally an arms-length activity. With portable form factors like laptops, netbooks, and tablets, computers become something consumers keep close to their body, and they use them in intimate places: The No. 1 place all three devices are used in the home is the living room, followed by an adult’s bedroom. 
• Abstracted to physical. The mouse/keyboard paradigm relies on an abstracted interaction with content. Touchscreens like those on smartphones and tablets enable direct physical manipulation of content in two-dimensional space. Cameras with facial recognition, voice sensors, and motion sensors like those on the Microsoft Kinect for Xbox 360 permit an even wider range of physical interaction with devices, where a user’s body and voice become the controller.

There are a host of technological innovations that make the post-PC era possible. Form-factor diversity enables computing in more contexts. Flash memory eliminates computing downtime. Wi-Fi and mobile broadband networks permit continuous connectivity. And cloud services support computing across multiple devices.

These technological innovations fuel social change, and vice versa. As people conduct more of their lives online—shopping, banking, entertainment—we require more computing in more places. The rise of social networking requires real-time connectivity to manage our relationships. And eroding work-life boundaries means that consumers demand devices that can do double-duty in their work and personal lives.

So where is this all going? In the post-PC era, the “PC” is alive and well, but it morphs to support computing experiences that are increasingly ubiquitous, casual, intimate, and physical. The new MacBook Air and Samsung Series 9 demonstrate PCs going in this direction. In the post-PC era, PCs are joined by smartphones and tablets, as well as future devices like wearables and surfaces. Imagine computing via a heads-up display embedded in your eyeglasses or contact lenses or learning about breaking news updates from a change in your electronics-embedded clothing. The products that will win have yet to be determined, but the underlying technological and social changes that will drive the post-PC forward are already here.