The tablet market has long been synonymous with the Apple (NASDAQ:AAPL) iPad. Cupertino created the niche in 2010, filled it with aplomb as quarterly sales soared to 14 million units, and made a pretty penny along the way. Challengers rose, only to fall, one by one. Still standing amid the bloodbath, the iPad was the only product line that mattered.

But I had to use the past tense in that introduction, because things are changing -- fast.

The market analysts at IDC just published a report on global tablet sales in the third quarter. The market grew 50% year over year to 28 million units -- but Apple's market share is slipping.

Seriously?
Apple shifted 14 million iPad units this quarter, and the 26% growth rate fell far below the overall market. The biggest winners were Samsung, Asus, and Lenovo -- Android vendors, one and all. Asus' big winner was the Google (NASDAQ:GOOGL) branded Nexus 7 and Lenovo is finding a respectable audience in China. Amazon.com (NASDAQ:AMZN) also sold 2.5 million Kindle Fire tablets, up from absolutely none a year earlier.

IDC notes, and I agree, that Apple won't stick to a pesky 50.4% market share forever. Cupertino fans held out for the just-released iPad Mini in the third quarter, thus shifting a few million expected unit sales into the holiday season. That's why the next quarterly update should return a few market share points to Apple's pocket.

On the other hand, Apple's rivals aren't resting on their growing pile of laurels. Google just introduced a new range of Nexus-branded tablets and smartphones. They were met with mixed reviews, but are priced to move -- the 7-inch version is $130 cheaper than the iPad Mini and the 10-inch model sells at the same $399 price point as Apple's budget-friendly but aging iPad 2.

One more thing
Then there's the new kid on the block. Microsoft (NASDAQ:MSFT) likes its brand-new Windows 8 software so much, it built its own tablet to run it. The Microsoft Surface tablet undercuts the latest full-size iPad in manufacturing costs but is priced right alongside Apple's tablets. Microsoft CEO Steve Ballmer insists that nobody has a tablet worthy of shining the Surface's shoes: "Not Apple. Not Google. Not Amazon. Nobody has a product that lets you work and play that can be your tablet and your PC. Not at any price point."

So he's betting on strong sales at any price, and the Surface will be very profitable if the bet pays off. If not, well, Apple has seen plenty of rivals slashing tablet prices until they sell. But by then, there's usually no profit at all left in the usurper's business model. Let's see if Microsoft can avoid that grim fate.

Finally, Research In Motion (NASDAQ:BBRY) is working up a whole new experience for BlackBerry fans -- but the refresh to BlackBerry 10 is still many moons away. RIM will miss the crucial holiday season, and it's awfully tough to start a new high-tech product line from scratch in a season where Valentine's Day is the biggest gift-swapping holiday. Somehow I doubt that BB10 will smell like roses and chocolate.

The tablet market is evolving, and Apple must be quick on its feet to stay relevant during this sea change.

Fool contributor Anders Bylund owns shares of Google, but he holds no other position in any company mentioned. Check out Anders' bio and holdings, or follow him on Twitter and Google+. The Motley Fool owns shares of Google, Microsoft, Apple, and Amazon.com. Motley Fool newsletter services have recommended buying shares of Amazon.com, Apple, and Google. Motley Fool newsletter services have recommended creating a bull call spread position in Apple. Motley Fool newsletter services have recommended creating a synthetic covered call position in Microsoft. The Motley Fool has a disclosure policy.
We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.