The best noise-isolating, in-ear headphones cut out all external sound - they literally seal your ears from outside noise, so you only hear your music.
Other advantages are that they're typically small and light, and require no additional power source - in fact they'll save your player's batteries, as you'll tend to listen at lower volumes, as you're not having to turn up your music to drown out external noise.
The cons are that not everyone likes the tight fit involved - this can range from discomfort to claustrophobia! Make ssure you try before you buy, and opt for a model that comes with a selection of earbud options, so you can find the very best fit for your ears (you can get custom-moulded buds from some manufacturers, too). The other con is that the sealing effect means you can hear yourself eat etc - that can make airline meals a tad freaky!
Noise-cancelling headphones, meanwhlle, generate soundwaves to counteract external noise: they include microphones that pick up that outside noise, and use battery power to generate the soundwaves to cancel it out. There are models in a range of sizes and shapes, with Sennheiser's PXC range the best performers in our opinion. They are especially effective at counteracting airplane engine noise, and there's none of the eating noise/closed-in issue as with in-ears.
Cons? They don't block out all external noise (though the best - like Sennheiser's PXC450 - get very close). They require batteries of their own (though again, you'll save player-power); and some people can hear the noise-cancelling soundwaves (describing it like a rushing noise).
Again, try before you buy!
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