In Sanskrit "Vajrabhairava" stands for 'Adamantine Terrifier'. Regardless which manifestation of Vajrabhairava you are looking at it he is always depicted as fear-inducing, scary, and intimidating. Not only is he terrifying to look at but - according to the Vajrabhairava Tantra - he also has conquered all evil spirits, including the Lord of Death, Yama. That's why he is also called "Yamantaka", the Slayer of Death. Depending on which manifestation of Yamantaka the Tibetans call him either gSin-rje-gsed or in the buffalo-faced aspect of Vajrabhairava rdo-rje 'jigs-byed. It seems that the term "Yamantaka" (and "Yamari") is used in a more general way than "Vajrabhairava" which is restricted to the buffalo-headed yidam of the Gelug and Sakya schools (see Lokesh Chandra). Sometimes the protector Kalarupa is called "Yamaraja" and comes as Outer, Inner, and Secret Yama- or Dharmaraja. In the Gelug school Yama- or Dharmaraja is part of the Vajrabhairava practice.
The Ngor Mandala collection of the Sakya tradition alone lists eight different forms/lineages of the blue/black buffalo-faced Vajrabhairava (which include the two Gelug ones) and four of red Rakta- or blue Krishna-Yamari (all without the buffalo head). All the former are yidams (=meditational deities) whereas Yamaraja (sometimes also called Dharamaraja) is a Dharma protector. Inner and Outer Yamaraja are blue/black, the Secret Yamaraja is red in color; Outer and Secret have buffalo heads, the Secret Yamaraja does not. There is also an emanation of Yamantaka (called Yamantaka orYamantakrt) in the Ten Wrathful Ones / Uncommon Protection Wheel in the Vajrabhairava and Guhyasamaja practices. Other emanations of Yamantaka appear as residents in the various mandalas (13-, 17-, 21-, and 49-Deity). They all embody the wrathful aspect of peaceful Manjushri; Yamnataka's consort, Vajravetali the wrathful nature of Saraswati.