I commented extensively in the thread that
Phil T referenced. Definitely worth a read for understanding basic specs.
I will add a few points to expand on that in the context of your basic question.
The basic rule of thumb guideline -
Take your budget, split it into two halves, set aside one half for the amp and another half for the speakers.
This isn't absolutely fixed. It just gets you started. In most cased, when all is said and done, most people spend a little less on the amp and a little more on the speakers. But a 50/50 split gets you into the class of amps and speakers you want. Then you can make adjustments according to your priorities and tastes.
Amp Power -
When comparing amps, make sure your are comparing apples to apples. There are many ways of rating power. One method to inflate power is to test it with 4 ohms speakers. That will make the power seem roughly twice as high as it really is. Also, amps are sometime rated at Dynamic Power, which is another inflated method.
The truest method is RMS power. It doesn't matter what RMS is or how it is derived. Just know when you see it, that that amp will absolutely kick out that amount of RMS power.
It doesn't so much matter which power ratings you use, as long as you are using the same method for all the amps you compare. Usually this requires going to the website of a few of the manufacturers you are interested in and getting detailed specifications. In some cases, you actually need to look at the Owner's Manual which are usually available on-line in PDF format. In the Manual, the detailed specs are usually on the last page.
In some cases, I compare identical amps in both the USA and in Europe, just to make sure I'm getting full and accurate information on power ratings.
Amp Features -
If you are looking at an AV amp or Surround Sound Amp, you want to make sure it has the features you need. If you need 3 HDMI (digital audio/video) inputs, then make sure your amp has them.
For example, you might have a digital TV cable box, and it may have one kind of digital output. Your video game might have a completely different kind of digital output. And you DVD/BluRay might have a third type.
If you have an assortment of input devices, then you need to make sure your amp can accommodate them all.
Next, you want to make sure the amp, as much as possible within your budget, has all the latest video encoding/decoding methods.
Typically, your budget will dictate how much amp power you can get, and how adequate that power is to the job at hand. Usually, the rated power is more than adequate. The question, though somewhat debatable, is whether the amp can actually deliver that power when you need it. But that is an argument we don't need to have now, and also, as I implied, it is dictated by your budget.
Speakers -
This gets dicey. In general there are three basic types of speakers.
'Egg/Pod' speakers, which are called this because they are very small speakers that frequently look egg-like or like odd space pods. I generally don't like them, but also acknowledge that there is a time and place for them. If you are in a small bedroom, or small dorm room, then having large speaker may be out of the question.
In some cases people choose them because they want something unobtrusive, or because they have kids, and can mount the 'Eggs' on the wall out of the way. You wouldn't want your little kid to pull a big heavy floorstanding speaker over on themselves.
'Egg/Pod' speaker run the full price range from under £200 to well over £1500 for a 5.1 speaker system. They do what they do well, and can sound good
for their size. Again,
for their size, they do what they do well.
Next, Bookshelf speakers, though fairly they do come in quite a range of sizes. This is probably the best bet and most popular type of speaker, and many would argue it is the best value for your money. The prices range from under £100 for a pair, to well over £3000 a pair.
Floorstanding speakers are larger multi-driver speakers in cabinets that stand on the floor, hence 'floorstander'; though some also call them towers. These can run from the equivalent of a bookshelf in a larger cabinet to very large multi-driver cabinet. The price range can be from about £150/
pr and up to infinity.
When looking at specs for a pair of stereo speakers or for front speakers in a surround system, I look for low frequency response of 60hz or lower. Below 50hz is better, and below 40hz is great. High frequency response specs are usually adequate and you don't have to worry about them.
HZ or hz or hertz means cycles per second, and is the unit used to measure frequency. So when you see 50hz, just think 50 cycles per second. kzh = kilohertz or a thousand cycles per second.
Now fairly, if the front speakers are for a surround system that will have a Subwoofer, they don't need to respond any lower than about 80hz. Though I personally like to have some reserve. The reason for this is that the subwoofer covers everything below 80hz.
Subwoofers (subs)-
Very large specialized speakers that cover the very low bass frequency range. In a 5.1 system, the .1 represents the sub. So, 5.0 means no sub, 5.1 means one sub (the most common), and 5.2 means two subs.
In most cases, the absolute highest frequency a sub can handle is a mere 200hz. Though it is typically set in the 80hz range according to the THX specification.
Some of the best subs go very low, down, in some cases, to 15hz. Though to be fair, you can probably only hear slightly below 30hz. But, you can feel things below 30hz and this is important to enhancing the movie listening experience. When the aliens land, you want to be able to feel the rumble of their warp drive engines.
More common consumer subs settle for response down in the area of 30hz and for most consumers that's enough.
'Egg/Pod' subs can be relatively cheap to match the cost of the matching satellite speakers. But good subs start at closer to £200 and go up to an easy £2000 each.
Now the bad news, depending on your budget, it is difficult to come up with a good AV amp and 5.1 speaker system for less than £500. And most would suggest that you spend at least £1000. But don't despair if that exceeds your budget. You can still get an acceptable all-in-one system for less, and many people are very satisfied with them.
That should get you started.
Steve/bluewizard