Its in the in-line remote of the headphone chord. It is rectangular in shape and the upper and lower end are reserved for adjusting the volume. The middle is for playing and pausing music and other media content. A 5 second Google search points to them having a battery life of a little over a month if you listen to them for a few hours everyday. Using them more will drain their batteries more.
64GB iPhone 5 | 64GB iPad mini | AppleTV 2 (2012) | AppleTV 2 (2010) | 2012 15" MacBook Pro, 1TB SSHD, 16GB DDR3 1600 MHz, OS X 10.8.3 Mountain Lion | Apple Lossless | iTunes AAC 192kbps VBR | iTunes 11.0.2| Library size = 1.04TB | Legacy iPods: 3G 40GB, 4G 40GB, 5G 60GB, 160GB iPod classic (2009)