When shopping for headphones, you should make sure that they are compatible with your mp3 player. You might also want to consider a wireless set. My favorite is noise cancelling headphones! You won't be able to hear anything except your music!

its an output but note it can also be an in put

The Markiplier's headphones are headphones that are designed and branded by Markiplier.

usful energy will be that it make sound for the person to hear through it

Supra-aural headphones are headphones that sit directly on the ears instead of enveloping them, which is what circumaural headphones do.