Quick Wall-wart (AC Adapter) overview:
Let's take a sample Adapter: one labeled as 5V, 500ma. There is a huge difference between what might be inside different adapters with this same rating.
The cheapest wall-warts have a transformer (hunk of metal with lots of fine wire wrapped around it; it converts the high-voltage AC to low-voltage AC), and 1 to 4 diodes (converts the AC to something kinda like DC). It may or may not have a filter capacitor on the output to somewhat smooth the output voltage. Unfortunately, they are unregulated - if the high-voltage AC varies by 10%, the output voltage will vary by 10%. What's worse, the output voltage will be much higher when no current is being drawn from the adapter, and much lower as you get near it's 500 ma rating. For our example adapter, it might put out something close to 9V if the wall-voltage is high and nothing is attached to it, or 4V if the wall voltage is low and you're trying to pull 500 ma from it. For these types of adapters, weight may indeed be an indicator of quality - a larger transformer weighs more and has less variation in output voltage as you change the amount of current you are drawing. For this style, you will seldom see one rated at more than an amp and a half.
A mid-level wall-wart is pretty much the same as the cheap one, except it has actual electronic circuitry at the output to regulate the output voltage. These will weigh more, and will probably have ventilation slots cut into the case. With this style, you can pretty much guarantee that a 5V adapter will be putting out 4.75V-5.25V, no matter what the high-voltage AC is (within the range specified on the label, 100-125V is common), and no matter what current you're pulling. Some older styles used circuitry that required it to geverate at least some current before it could regulate; you might see 7V out of it if you don't have anything connected. Once again, you will seldom see one rated higher than an amp and a half. Almost no new devices will have this type, though you can find a lot of these on older devices and in surplus catalogs.
A high-level wall wart doesn't have a heavy transformer in it, though it will have a good-sized inductor in it (much, much lighter than a transformer). It has electronic circuitry that converts the high-voltage AC into low-voltage AC in a completely different way (don't ask how; just take it as magic), and also regulates the output voltage. These can easily be built to accept a much wider range of input voltages (for example, 100-240V AC, allowing it to be used pretty much anywhere in the world). The output voltage will be in the 4.75-5.25 volt range no matter how much current you're pulling out of it (as long as you don't exceed 500 ma). These can be bought with ratings in the 4-5 amp range. These cost more than the el-cheapos, but some manufacturers use them because it can make their electronic device cheaper (you don't have to worry about building a voltage regulator into your device), they are much lighter (saves on shipping costs), and they are attractive to travelers (you don't have to worry about carrying voltage converters around everywhere, though you still have to have plug converters).
And thats the truth...
/frank
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