Usually these are worn on your chest. For the last several years only chest rate based straps had any level of accuracy for exercisers, but lately some new entries which work in your ears or on your forearm are hitting the market.
A great device is the Wahoo Tickr, available here: http://www.amazon.com/Wahoo-TICKR-Monitor-iPhone-Android/dp/B00INQVYZ8
A capable and lower cost option is the 60beat Blue, available here: http://www.amazon.com/60beat-Heart-Monitor-iPhone-Androids/dp/B0096DM3ES
Some iOS devices do not have Bluetooth Smart support, so to use this feature you must have one of:
- iPhone 4S, 5, 6 or newer
- iPod Touch 5th gen or newer
- iPad Air, Mini, 3rd & 4th gen or newer.