Kapalabhati is a yogic breathing practice. It is not exactly pranayama, but more of a Kriya type technique. Pranayamas are usually done during or after Asana practice. Kapalabhati though is normaly done before or separate from other practices.
Kapala in Sanskrit means "skull" and bhati means "to shine". This way it is Skull shining breath. It is called this way as it purifies the whole system, clears Nadis (subtle energy channels), directs Prana upwards. It cleanses and energises the nervous and circulatory system and promotes clarity of mind and focus.
Kapalabhati is done in a sitting position. Body and mind relaxed. Focus is on the abdominal and Manipura area.