Apple didn't invent the notion of mobile apps—and indeed the company completely ignored this possibility for its otherwise forward-leaning iPhone when that product first shipped in 2007. But thanks to rampant customer complaints, Apple finally changed its mind and, in doing so, ushered in a new age of apps and app stores. Most impressively, to me, is that the success of these apps flies in the face of the conventional wisdom that all major software development is now occurring on the web. That's true, but only if you ignore the mobile market, where tiny, proprietary apps are flying off of virtual shelves and providing users with targeted fun and functionality. And the best part, for developers: Unlike with most web apps, they can actually make money selling apps for mobile devices.

The year 2010 wasn't the year mobile apps first appeared, and it wasn't the year that mobile apps finally took off. But 2010 was indeed the year of the app, and the app store, and it's now not possible to launch a new platform of any kind without providing an app store for it. This is why apps—and more to the point, app stores—define 2010 more than any other technology does. App stores are so important that traditional platform vendors such as Microsoft and Apple are now racing to provide them for their desktop OSs as well. This is the surest sign yet that they see the writing on the wall: The future of software retailing has nothing to do with physical stores and everything to do with discovering and acquiring new functionality, electronically, and over the air.

So app stores are now ubiquitous. But they also come with a new definition, of sorts, for what apps really are. In traditional desktop OSs, applications are typically huge, monolithic applications such as Microsoft Office or Adobe Photoshop that take a considerable amount of time to install and configure. These applications are typically tied to a single PC, as is Windows itself, and a user would need to purchase multiple copies to install an app on multiple PCs. Updating is manual and typically occurs on an application-by-application basis, separate from any centralized OS updating functionality.

Apps—as opposed to full-blown "applications"—tend to be smaller, simpler, and easier to find, deploy, and update. They typically serve a single function and don't usually branch out into multiple areas of functionality, as Microsoft Office does. There are many free apps, of course, but also a wide range of very inexpensive apps, some costing just 99 cents, pushing users into a micro-payment model in which a number of very small purchases are combined in a single bill.

Apple's model for its iPhone (and iPod and iPad) App Store is so successful, so well done, that everyone is copying it. We're seeing app stores for competing mobile platforms such as Android, webOS, BlackBerry, and Windows Phone 7. Even the ancient Windows Mobile OS, set for extinction, was updated with its own app store late last year in order for Microsoft to establish a presence there and gain valuable experience.

But app stores aren't just limited to mobile platforms. Google has an app store for its Chrome web browser (and coming Chrome OS). Amazon has an app (and book) store for its Kindle devices. Apple is launching an app store for Mac OS X next month, and Microsoft plans one as part of Windows 8.

App store-like functionality is also available in unexpected places, and with humongous software programs that one might think aren't necessarily a great fit for this model. You can purchase an Office 2010 suite or standalone application from the Microsoft Store online, for example, and then download it to your PC and install it locally, instead of waiting for the company to send you a disc in the mail. And if the $499.99 price of an electronically downloaded Microsoft Office Professional 2010 doesn't cause you to blink, consider this: You could easily spend upwards of $1,800 on various software products at Adobe's online store. Heck, the Creative Suite 5 Master Collection will set you back a whopping $2,600. And yes, that's for the downloadable version, all 11GB worth.

Thanks to the rise of the app store, electronic software delivery is no longer the future—it's the present. And while this situation was likely inevitable (did we really ever jump in a car and drive to a store to buy a shiny disc that we would then drive home with and install on our PCs?), it's happening now. And it's only going to get better and more seamless as we move forward. And for that, we thank the app store. It's a big idea, bundled into an impossibly cute and usable package. And it’s the single most important technology trend to come out of 2010.