Buddhism does not have a particular leader as such. Some sects, such as the Gelugpa, recognize the Dalai Lama as the spiritual leader of their lineage. Sects can recognize certain individuals as reincarnations of significant people in the lineage and therefore show them the respect and accept the guidance of the person, however, they are not required to do so.
When the Buddha was dying, he was asked who would lead after he died. The Buddha said that no single person would lead; that the teachings and training that he provided to his students would serve them, and that no single individual would be the source of leadership. This is thought to have been to ensure that no single interpretation or viewpoint would allowed to dominate, and it is believed it was intended to avoid Buddhism becoming too rigid, since the whole point of Buddhism is lack of rigidity and attachment to ideas. Correction The leader of the Gelugpa sect is not the Dalai Lama, but the holder of a different office called the Ganden Tripa, who is the lineage holder of the throne of Ganden Monastery, the reincarnation of Je Tsongkhapa in his aspect as the founder of the Gelugpa and of Ganden itself. However, most Tibetans recognise the Dalai Lama as an even higher being. Although he has no formal authority within the other Tibetan sects, since the "Dalai Lama" reincarnation is traced back only to Gedrubje, one of Je Tsongkhapa's direct disciples, he is widely regarded not only as the spiritual and political ruler of Tibet but as the supreme living incarnation of Buddha Avalokiteshvara (a.k.a. Chenrezig). In general, each Buddhist's highest authority is his or her own personal choice of Guru or Gurus.