SUMMARY:  As a friend recently told me, there are two things you need to know about getting a UPS for your computer.  1: You need one.  2: Use a shopping cart when you buy it.  They're heavy.

Always get a UPS (Uninterruptible Power Supply) for your computer, unless you just really don't care about it or the data you save on it.  I found out the hard way just how important a UPS really is, back in early 2000.  I was running an AMD K6-2 350MHz system with a 4GB drive.  That's anemic at best by today's standards, but at the time it wasn't bad (and even now it's still a reasonable set of specs for a Linux desktop system).  I had it plugged into the wall via a surge protector in my apartment in downtown Riverside, California, with no UPS.  One day, my computer suffered a catastrophic hard drive failure.  Specifically, the hard drive suffered a head crash.  That means at least one read/write head turned into a plow, dragging along the surface of a drive platter and gouging furrows in it.  If you look at a platter to which this has happened, you may not even be able to see the scratches, but on the level at which data is saved it's horribly destructive, and any data you have saved is pretty much screwed, and you could conceivably spend tens of thousands of dollars trying to recover it if you don't have backups.

Luckily, I had just backed up the data saved on that drive to CD-RW about a week or so before that, and hadn't made any additions to the data that I couldn't afford to lose with the exception of some stuff I also happened to have on floppy.  The major problem, then, seemed to be the need to replace the hard drive.  I scrounged up a good drive, and I tried it.  Unfortunately, the modem failed (I was using dialup Internet at the time, of course), and I needed that to work.  I got a new modem and, next, the motherboard failed.  When I replaced that, I realized that the power supply unit (PSU) must be producing "dirty" power, causing components to fail, so I replaced that too.  I ended up having to replace a stick of RAM, as well.  Of course, the question that then arises is this: Why was the PSU outputting power fluctuations to the computer's components?

Downtown Riverside suffers some flaky power.  The electricity coming from the outlets fluctuates a bit.  That's true of any place with AC power outlets, of course, but moreso of Riverside than some other places I've lived.  Over time, these spikes and drops in power will gradually degrade the power supply unit, which will then start feeding similarly unconditioned power to the computer's internal components (primarily drives, motherboard, and processor).  Because these components include very delicate parts that require carefully metered, steady power to work properly, minor fluctuations can over time cause a lot of damage.  Eventually, computer parts start failing.  A good UPS receives AC power from the wall in its raw form, then "conditions" it (either through a delayed direct-conditioning process or by using wall power only to charge the battery, and running the equipment off the battery itself) so that your computer won't receive the fluctuating, unsteady power we've come to expect from the utility companies.

Even if you're not worried too much about having to buy new hardware and don't have any data about which you care at all, you might still have a problem.  With the way Microsoft is going, even changing a couple of components in your computer can cause an instance of Windows XP to cease functioning because of the way Microsoft is trying to take control of how people use computers.  The aggravation of having to call up Microsoft and tell them you need a new product key so you can activate Windows properly, as if you needed permission to use something for which you've already paid, can be avoided in part by ensuring that your equipment is somewhat protected against failures due to power-related degradation.  With that added to the issues with hardware failure in and of itself, it should be fairly obvious that a UPS should not be considered optional for most computer users.

Part of the problem with recognizing how important a UPS can be is the simple fact that gradual PSU failure is typically very hard to diagnose because it can have unpredictable effects on the operation of your computer, and any problems it causes are usually diagnosed as failures of other specific parts.  Those other parts are indeed failing, of course, but if you just plug in a new one to replace it without addressing a PSU failure, the new part will probably fail faster than the old one did.  Even if the PSU failure is correctly diagnosed, people usually don't think in terms of preventing the next such failure because they don't realize that it was probably caused by power fluctuations coming from the wall socket -- they only think in terms of replacing the power supply unit with a new one.  In time, the new PSU will likely suffer the same fate as the old PSU, then.

Now you know the "why" of buying an uninterruptible power supply.  Next you need to know the "how" of buying a UPS.

First of all, at least get a "line interactive" unit.  They tend to cost a little more than a mere standby unit (Standby Backup Offline, or SBO), but they're worth it.  In fact, with a standby backup unit, you basically lose all of the important benefits of a UPS for protecting your computer.  The term most often used to refer to the added protection provided by a line interactive UPS is AVR, or "Automatic Voltage Regulation".  Even better than a line interactive UPS, though, is an "online" UPS.  This provides the equivalent of firewall protection between the outlet and your computer.  It doesn't just offer "conditioned" power: it actually feeds your computer only power from the same level (in an at least three-stage process of conditioning power) as the battery with all the carefully controlled conditions for power output as you'd have if actually running it directly from the battery.  I unhesitatingly recommend an online unit, if at all possible, and recommend avoiding an SBO unit because of the fact that it's worth little more than a false sense of security and $40 or more of your hard-earned money.

The amount of standby power time provided by a UPS is quite variable, depending not only on the model of UPS you buy, but also on what you have plugged into it and what your computer is doing while it's plugged in.  Most consumer UPSes come with a CD with software on it that can be used to make your computer interact with the UPS so that, in the case of a full power blackout, the computer can shut itself down "safely" before the UPS battery power runs out.  This, of course, is an extension of what uninterruptible power supplies were originally meant to do, but for my purposes it has always been the least important function of a UPS: my major concern is ensuring that my hardware isn't damaged.  While this software that comes with many UPSes usually works well with Windows systems, I have yet to see a consumer-level UPS come with such software for a Linux or other (non-Mac) unix system.  This doesn't mean the software doesn't exist, but you'll have to find it elsewhere.  If you're a Linux user, you probably already had plans to start looking for information on the matter before the end of the previous sentence, but it's worth mentioning that you need to research UPS brands and models to see which are well-supported by Linux before you buy one for use with a Linux system.

Look the box over on any UPS you're considering buying.  It should say something on it about equipment guarantees and insurance.  You'll have to determine how important this is to you, and how to weigh it against physical characteristics of the UPS, but keep in mind that some UPS brands are actually effectively "insured" against equipment damage from the moment you purchase them, without having to fill out any forms.  This is usually preferable when dealing with consumer-level uninterruptible power supplies.

Another factor to consider that depends entirely on you is the number of outlets on the thing.  Keep in mind that most UPS models have several outlets connected to the battery backup and several that are nothing more than "surge protection", which is almost useless for protecting a computer.  Some might provide some measure of power conditioning even on the sockets that don't access the battery backup, but that is rare, and it still doesn't protect you against blackouts.  The important outlets are those that actually provide battery backup capability.  While some of you may be thinking you can just plug a multiple-socket extension cord, surge protector, or other power line extension into a battery backup socket on your UPS, this is usually a bad idea because of the limited output of which a UPS is capable.

Each UPS is only rated to be able to output a certain amount of power.  If you exceed this, depending on your UPS model, you may lose power entirely, suffer a consistent brownout effect, drain the battery even when there isn't a blackout, or otherwise compromise the effectiveness of your UPS.  The rating on a UPS is almost universally expressed in VA, which stands for "Volt Amps".  The VA capacity of your UPS corresponds to the number of watts or amps of power draw of equipment you can plug into the UPS without causing a problem.  Check what the power draw of your equipment is before shopping for a UPS.  If the number given is in watts, multiply the number by 1.4, and buy a UPS that exceeds the resulting number by a comfortable margin.  If the number is in amps, multiply it by the nominal line voltage in your area which, in the United States, is 120 volts (in much of Europe it is 220 to 240).  Most consumer UPSes range in capacity from 350VA to about 1400VA, with line interactive UPSes starting at about 700VA.

Finally, look into what the UPS uses to connect to your computer for management of power (doing controlled shutdowns, et cetera).  If that's a feature you're interested in using, ensuring that it will connect to your system in particular (via USB, serial cable, Ethernet, or other means) will be an important consideration.  If not, don't worry about it, I suppose.  It's up to you, just like everything else pertaining to your computer's stability, security, and physical protection.