At first glimpse it does not seem a very complicated question. There is not much to it:
Nozzle with brush, motor with impeller and filter, etc.
When you take it a step further and consider other parameters it gets a bit more complicated:
Power consumption, suction, airflow, particulate count, noise, weight, etc.
Oh yes, it can be even more confusing:
Capet and/or bare floor vacuuming, upright, canister or central, ergonomics, attachments, cost of consumables and the vacuum itself, warranty, life expectancy and finally - is this the vacuum thought you wanted to begin with? And for dessert, try throwing everything in one pot and shaking it well. You will end up with thousands of variations that you have to pick from considering your needs and financial resources.

Vacuum cleaner manufacturers are not making it any easier for us either. To increase their profits they can go three ways:
    1. Reduce cost
    2. Increase price
    3. Both of the above
You guessed it - the most popular is option #3. The biggest problem in this picture is that to increase price while reducing or at least maintaining the cost rarely can be done by spending a lot of money improving the product itself. Even after big improvements the manufacturer still has to spend a lot of money to advertise improvements to the public to prove their vacuum's advantage over competitor's vacuum cleaner. Therefore, it is pretty common to just skip product improvement and jump straight to marketing: shuffling test results around and making various claims. One manufacturer may start advertising awesome HEPA filtration while failing to disclose or even test how much dust is escaping into your room from under the nozzle. Then another manufacturer will try to charm you with a 12 amp motor or 100 inch water lift while it may not necessarily result in better overall performance. It may get even worse - they will send a salesman to your house that will subject you to a 3 hour sales pitch and will not leave unless you sign a purchasing contract.

But do not give up just yet, we have some really good news - a lot of $200-$300 vacuums are quite capable of getting the dirt out of your house. The most important factors are:
    1. Choosing appropriate type of a vacuum to fit your specific cleaning needs.
    2. Vacuuming your house before you get sand scratched varnish on the floor or dirt buried under the carpet from where no vacuum will pull it out.
We will not tell you it's about time you do some cleaning in the house but we will try to help you to untangle the parameters and some features so you can make a more educated decision before buying your next vacuum cleaner.



What does PHP or Peak Horse Power of the vacuum cleaner mean?
This parameter has been used for a while for marketing purposes. Maybe because one can somewhat relate to it since horse is a big strong animal that can pull a huge metal plow so a one (or even more) horse power vacuum should not have a problem cleaning carpeting. Horsepower was derived using results measuring maximum load a vacuum cleaner motor can take without burning. This parameter may make sense when dealing with drills, saws, lathes and maybe some other tools but it has very little to do with the performance of a vacuum cleaner. Nowadays you rarely will see vacuum cleaner horsepower rating anywhere.



What do vacuum amperes and watts mean and how important are they?
These terms represent the maximum measure of power of electric current flowing to your vacuum cleaner. Maximum amperage allowed for an appliance is 12 amps. The most important amperage is of the main vacuum motor as it is what drives the fans. All other draws on the power such as light bulbs and the nozzle motor are not directly involved in creating suction and airflow. Watts represent the power of the vacuum that can be derived from amperes and voltage.
Watts = Volts x Amp
Voltage in all outlets is the same. For USA it is 120 so plug into the formula:
Watts = 120 x Amp
From this you can easily calculate how much it will cost to operate the vacuum
Watts / 1000 = Kilowatt
Look at your electric bill and see how much are you paying for 1 kWh (kilowatt-hour)
"Vacuum kilowatt" x "Cost per kilowatt-hour" = "Cost of operating the vacuum for 1 hour"
However, amperage is just as important an indicator of the performance as gasoline consumption is an indicator of the performance of a car - it more or less indicates the capability of the motor but it is more common to find very poorly designed vacuums than a cars. An engine consuming less gasoline may perform better combined with a good transmission, appropriate tires and other components to make that car go faster, quieter and longer distances than a  bigger guzzler. Same with the vacuum - good fan system design, filter allowing good airflow, nozzle/brush design may make a vacuum cleaner "go a lot more miles" with less kilowatt-hours of electricity consumed.



What exactly is meant by vacuum cleaner suction?
It is an important vacuum cleaner performance parameter usually measured in inches of water lift.
 A "U" shaped tube with both ends open is filled with water (Figure 1). To be exact water has to be 4 degrees Celsius or 39.2 degree Fahrenheit to achieve the standard viscosity. Then to one side of the "U" a vacuum is attached and turned on. The vacuum lifts the water raising the water in one leg and lowering water level in another (Figure 2). This difference is measured in inches and this is the measurement ("h" in Figure 2). It also easily can be converted to other units. This is an example of just a few:
1 inch of water = 0.073 555 912 462 inch of mercury [0 degrees C]
1 inch of water = 0.036 127 291 827 pound/square inch (PSI)
1 inch of water = 0.002 458 316 148 5 atmosphere [standard]

1 atmosphere [standard] = 406.782 504 61 inch of water
1 inch of mercury [0 degrees C] = 13.595 100 197 inch of water
1 pound/square inch = 27.679 904 843 inch of water

Very important - no vacuum looses suction even with nearly full bag or nearly clogged filter. As long as some air can pass through the filter medium it will generate just the same suction  at the tip of the hose. They keyword is "sealed suction" and it remains the same. While filters are full / clogged - another very crucial parameter is being affected - AIRFLOW. Airflow is another very important factor that defines sweeper capability of lifting that particle from your carpet or floor and putting it in the bag.



What is vacuum cleaner airflow?
Airflow in the vacuum cleaner industry is measured in cubic feet per minute (CFM). This is exactly what it sounds like - measure of how many cubic feet of air a vacuum cleaner moves through the system per minute. It is an important factor but not the sole indicator of sweeper performance. Airflow will be obstructed by a clogged filter. Just to illustrate - imagine you measure the suction with manometer completely sealing the opening on the hose with the gauge. Airflow will be "0" while suction measured will be just like in manufacturers spec sheet. If you start vacuuming carpet - airflow will be there and it really depends on the strength of the suction if air will be going fast enough to lift that particle off the rug and put it in the bag or another type of filter.



What is "air watts" ?
This is yet another attempt of the vacuum cleaner industry in rating vacuum cleaner efficiency. I would say that it is an honest attempt as it ties airflow and suction together but due to complexity and differences of various cleaning systems it is not the only indicator of overall performance of the cleaning system. The formula calculating air watts is " airflow (CFM) x suction (inches of water) / 8.5"



What is good filtration for a vacuum cleaner?
Nowadays HEPA has become standard for vacuum cleaner filters. Vacuums that use cyclonic action, water filters or regular paper filters usually have HEPA (High Efficiency Particulate Arrest) filter on the exhaust. For a filter to qualify for the HEPA label it has to capture 99.97% of airborne particles 0.3 micrometers in diameter. In general - HEPA is a good filter. Problems arise when HEPA filter has small area and/or primary filter lets too much dust that clogs HEPA cartridge very quickly. Clogged HEPA becomes airflow obstruction and diminishes a sweeper's cleaning capability.This is why some vacuums (especially cyclonic) have a sponge filter between the cyclonic container and HEPA filter. In a well designed system, pre-filters would retain just enough dust to not clog the HEPA filter too fast and it works out well.
Take a look at the size of pre-filters (for bag systems) and if it seems very small you may want to change it fairly often just to maintain good airflow. Be sure to check prices for filters and bags. Most companies have them reasonably priced but some may surprise you with filter prices.
Another thing to keep in mind is air that never goes through the vacuum cleaner - never gets filtered. Some vacuums may have poorly designed intake nozzles (power nozzles) that allow dust into the air right right from under the nozzle.
Filtration tests are fairly complicated and expensive. There are many variables that have to be kept the same: airflow rate and terminal test pressure, test carpet, test dust (yes there are various size test dusts - it is not as simple as just vacuuming any rug). Filtration ought to be measured in an enclosure to take into an account dust that is raised into the air from under the nozzle while using the vacuum. We have seen some videos on Youtube.com where just a gauge is used to test vacuums measuring how many certain size particles pass through a filter. We believe that all above described variables not being equal would greatly influence test results especially if test is performed not in the same spot the same day and with brand new vacuum cleaners.



Is the sweeper easy or hard to learn to use ?
The vast majority of vacuum cleaners are "Plug and Play" type of devices, however others get more complicated. Making them more versatile and useful for a wide variety of cleaning jobs usually makes them a bit more "tricky". If you see a light, a button or a lever on your newly purchased vacuum - refer to the owner's manual and see what is it for. Some vacuums are equipped with clog indicators, others need certain things done in certain sequence to go from using sweeper as upright to vacuuming with the hose. Believe it or not - this is pretty common for people to bring their vacuums to a vacuum shop because they "do not suck" and only then realize that there is another filter that was supposed to be replaced a year ago. Flip through that book - this is the only time you have to do it for entire time you will have this vacuum.

How comfortable is the vacuum to use ?
Things to look at: