Lots of consumers are planning to buy tabletcomputers this year, and lots of companies are hoping to sell them. Apple managed to sell around 15 million iPads last year in nine months, and, for many users, the iPad has replaced the laptop, at least for some uses. So it's no surprise tech companies are planning to roll out as many as 80 iPad rivals in 2011, by some estimates.

But the tablet mania can be confusing. The coming devices will be heavily defined by a variety of operating systems they'll use. They will be offered in different screen sizes, with attendant pluses and minuses. And they'll come from very different kinds of companies -- major computer makers like Hewlett-Packard, and Dell; phone makers like Motorola and Research in Motion; multi-faceted electronics giants like Samsung; and even Vizio, which largely makes TVs.

And, of course, a second generation of the iPad is expected to be announced in the next few months.

Here's a guide to what to expect in the tablet market in 2011, and some key factors that could affect your choices. As it's early in the year, the road map is necessarilyincomplete. Prices aren't generally known, though many rivals will be trying to undercut the iPad's $499 base price. Some will be sold on a subsidized basis through phone carriers, others won't. And there will surely be surprises as companies adjust their strategies.

Given the quality and success of the iPad, it makes sense for tablet buyers to hold off until they see what Apple has up its sleeve for the second version. One big reason: The iPad has a huge head start in third-party apps designed for tablets -- more than 60,000 of them, plus the 350,000 or so iPhone apps that the iPad can run. But Apple is more secretive than the CIA, so we know little about this product. I believe it will almost certainly have one or two cameras, and be able to make video calls. And there's widespreadspeculation that it will be thinner and lighter; the original's 1.5-pound weight was a bit too heavy for extended use for some. There's some evidence it will have at least one added port, perhaps for a camera memory card or connection to a bigger display.

Just as in the smart-phone market, the bulk of Apple's tablet competitors will rely on Google's Android operating system, which is provided free to hardware makers. Most of the hardware companies mentioned above are counting on Android to allow them to undercut the iPad on price, add different features, and attract third-party apps.

The big question mark here is the tablet-specific version of Android that's code-named Honeycomb, which hasn't been publicly unveiled. The first Honeycomb tablet is likely to be a 10' model called the Motorola Xoom, which is expected to show up in early spring. The others will mostlyemerge in the summer. If Honeycomb succeeds, the Android tablets could be a very attractivealternative, though it will take awhile for large numbers of third-party tablet apps to become available. Honeycomb will support Flash video on the Web, while the iPad doesn't.

One big issue will be how these Honeycomb-powered products will be differentiated from each other. Price and hardware could be decisive. Speed, size, screen quality, connections to TVs, and support for fast, 4G wireless networks are all possibilities.

BlackBerry-maker RIM plans a light, thin, 7' tablet called the PlayBook, likely in the next few months. In demos, it looks handsome and colorful -- nothing like a BlackBerry phone. That's because it runs on an entirely different operating system.

One unusual feature of the PlayBook is that, in key respects, it's more of a companion to a BlackBerry phone than a standalone tablet. It draws its cellular connectivity from a BlackBerry, rather than having it built in. The first model will lack its own email, calendar and contact apps, and instead merely view and interact with those in a user's BlackBerry. This could be a plus for BlackBerry users -- and a downside for users of other phones.

H-P plans to unveil a 10' tablet on Feb. 9 based on Palm's sleek webOS operating system, which H-P now owns. Based on trademark filings, it's likely to be called the HP TouchPad. While the computer giant has said little or nothing about the device, it's likely to ship this summer and feature, out of the box, integrated video calling and document editing. A big question is whether the software scales well to a tablet size and whether third-party developers will write enough apps for it.

Unlike the other players, Microsoft seems to be planning to cram a full PC operating system into a multi-touch tablet. The first Windows tablets, which will be out soon, will be based on Windows 7, use styluses, and be aimed mainly at corporations. Even their makers privately express little enthusiasm for them. Later in the year, Microsoft is expected to roll out a new Windows-based multi-touch tabletplatform better designed to go head-to-head with the iPad and Android tablets.

Consumers will have to decide whether they like the 10' size of the iPad, and of many of the new Android tablets, or the 7' size of some other models. A 7' screenactually has less than half the surface area of the iPad's display. But 7' tablets -- like the Samsung Galaxy Tab -- are lighter and easier to hold in one hand. They also can cost less. One big-name PC maker has been working on a 4.8' tablet. Since the iPad lacks a built-in physical keyboard, and common PC ports like USB connectors, many competitors will try to outdo it with these things. Lots of them will have some form of USB port, and a few will come with keyboards that slide out or fold out somehow. Lenovo plans to ship an Android tablet that can be used as a slide-in screen for a Windows laptop.

All this tabletcompetition is good news for consumers, but study the landscape carefully and weigh your options before plunging into the new category.

Walter S. Mossberg