Lavalier microphones are also known as lav, lapel or lap microphones. A lavalier mic is a very small condenser mic designed to pick up speech from a single person.
Lavalier mics are usually attached to the subject's clothing with a specialised clip. Obviously the preferred position is on the lapel or thereabouts. This provides consistent close-range sound pickup and is ideal for interview situations in which each participant has their own mic. It also means the subject doesn't have to worry about mic technique.
Try to discreetly hide the cable in the clothing (bearing in mind the notes below). If there is nowhere to place the mic on the subject's chest, try the collar.
Lavalier mics can be quite susceptible to noise caused by movement of the subject. Position the mic securely, making sure it won't rub against the clothing. Ask the subject not to move around too much, and make sure the cable won't be pulled if they do.
A small wind filter can be used to reduce wind noise.