The Karmapa (honorific title His Holiness the Gyalwa Karmapa, sometimes spelled Gyalwang Karmapa or known as the Karmapa Lama) is the head of the Karma Kagyu, the largest sub-school of the Kagyu (Wylie: bka' brgyud), itself one of the four major schools of Tibetan Buddhism.
The historical seat of the Karmapas is Tsurphu Monastery in the Tolung valley of Tibet. The Karmapa's principal seat in exile is the Dharma Chakra Centre at Rumtek Monastery in Sikkim, India. His regional monastic seats are Karma Triyana Dharmachakra in New York and Dhagpo Kagyu Ling in Dordogne/France.
Due to a controversy within the Karma Kagyu school over the recognition process, the identity of the current 17th Karmapa is disputed. See Karmapa controversy for details.
Example video: His Holiness the 17th Gyalwang Karmapa visits Harvard University.
Example via www.ramdass.org: H.H. the 17th Karmapa
Example via www.ramdass.org: The 16th Karmapa, Rangjung Rigpe Dorje