Headset microphones are mounted on to a pair of headphones or earbuds, usually in a position that makes them closer to the mouth of the user. They often come cheap or free with modern headphones as they are an integral part of the noise cancelling system. Having the microphone physically attached to the user is a great way of making sure they don't move to far from the microphone. They can be dynamic or condenser, with most consumer earbud microphones being electret condensers.