The little black thing singers have in the backpocket?

Question

And why do they have ear phones in.

example:

http://www.youtube.com/watch?v=HLHs6EXbZxs&feature=related

Answer 1

That is a radio transmitter/receiver, and the batteries that power it.  It can do many things, which features are used, and how, is usually up to the musician.  It is sometimes used to send signal from a wireless microphone.  The earphones you see are also connected to it, and again, depending on the users preferences, will produce the overall sound from the band, just the sound produced by the artist using it, or any combination.  Bass players and drummers, or singers singing in harmony, often like to share a channel for feedback.  The person running the mixing board will also sometimes have the option to communicate during live performances with the artists individually or as a group that way.  "Move away from the speaker, I'm getting more feedback than the gate can suppress", etc.

Answer 2

it's for their mic.  it's the battery and it's how they control it off stage.  At least thats how it works for my school's body mics.  And the ear phones are so they can hear themselves (i believe), because a lot of times concerts are very loud and the singer can't hear themselves singing.

actually now that i watched the video it looks like the battery pack is for this ear piece, since he is singing into a standing mic.