Questions:
1. can anybody tell me what is the difference between a free-air sub and a non-free air sub?
2. Should the speakers have a higher power capacity than the amp. I heard this should be the case, so that the speakers won't blow if there is an accidental surge of power from the amp?
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Tomasito
A free air sub doesn't need an enclosure. You can mount it on a board or rear deck. Going by RMS power, yes it usually is a good idea to get the subs that can handle more power than the amp can provide.
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'85 Toy
Free air bascally uses your trunk or car as the enclosure. Like supradude said you can mount those on a board or your rear deck and use the trunt as the enclosure. When a sub is used in free air it can't handle a lot of power either. Free air usage is very hard to master (just thought I'd add that). About the speaker Rms being higher than the amp RMS, that all depends. If you under power your speakers they will generally last longer. You also reduce the risk of blowing your speakers. Some people run higher RMS. SPL competitions pump more watts into speakers than most people would want to. Regular listeners sometimes do too. All you have to do is make sure you set the gains right. If you are knowledgeable enough, then you can pump a lot more watts into a sub without a problem.
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That's my opinion. Take it, leave it, or correct me.