HP released its Slate 500 tablet this week. Immediately, everyone started comparing it with Apple's iPad. But the two devices have nothing significant in common. They are in entirely different device categories and can even be thought of as opposites.

Some of my fellow journalists, industry watchers, Wall Street types and others seem to have difficulty making this distinction and continue to confuse the public by comparing the two.

I believe there's an important distinction -- as important as the difference between, say, a PDA and a smartphone was back when PDAs existed.

There will be many devices available in the same class and category as the iPad, and there will be many similar to the Slate. If you want to make sense of the new mobile market, you must understand the difference between the HP Slate and the Apple iPad.

Here's how to make that distinction.

Slate is a motorcycle, iPad is a bicycle

If you think of computing devices as vehicles, with servers being like trucks and PCs like cars, it's easier to understand tablets. The Slate is like a motorcycle and the iPad like a bicycle.

The motorcycle, like the Slate, is more powerful. That doesn't mean it's better. Which is more versatile, functional and usable by the widest range of people? Which one can you take on a bus, or hang inside an apartment? Which one is more likely to be used by children, the elderly and people in small villages around the world? Which is easier to maintain? Which is easier to use? Which is more energy-efficient?

You could argue that a motorcycle is "better" and "more powerful." But how many motorcycles do you have in your garage, and how many bicycles? There are about 200 million motorcycles in the world, but more than 1.4 billion bicycles.

If you can accept this analogy, then you can understand why it makes no sense to even mention the iPad when reporting the Slate's availability. When a new motorcycle comes out, the motorcycle magazines don't ask, "Will this kill the mountain bike?" It would be absurd.

Beyond metaphorical comparisons, what are the actual differences between HP Slate-type devices and Apple iPad-type devices? The differences are of class, interface, generation, usability, market, application model and vision. Let's look at each.

The class difference

The Slate is a PC. The iPad is an appliance.

The Slate is running the same operating system as your desktop PC and laptop, assuming you're a Windows 7 user. It's running components designed for PCs, including eight times the amount of RAM that's in an iPad. It runs PC applications unmodified.

The only difference between a Slate and a PC is that with the Slate, the screen can be used as an input device; a mouse and keyboard aren't required. But if you plug in a mouse and keyboard, everything will work fine. There are hundreds of different scenarios for PC input; the HP Slate is just one, and not a particularly exciting or innovative one.

Apple's iPad, on the other hand, is neither a PC nor an alternative to a PC. You use it in addition to using a PC. It's an entirely different class of device designed from the ground up to function as an information appliance.