As I discuss in this answer, each of the four Vedas consists of four portions: Samhitas, the core part of the Vedas which consist of verses heard from the gods; Brahmanas, which provide instructions on the proper conducting of important rituals; Aranyakas, which provide a guide to rituals meant for forest-dwellers and hermits; and Upanishads, which consist of conversations between teachers and students which clarify the philosophical message of the Vedas. With the Yajur Veda, however, there are complications.
First of all, the Yajur Veda comes in two versions, Krishna or "dark" version and the Shukla or light versions. The Krishna Yajur Veda has complications of its own: as I discuss here, the Taittiriya Samhita and Taittiriya Brahmana of the Krishna Yajur Veda each contain both Samhita and Brahmana portions! The Shukla Yajur Veda is a bit cleaner in its division (which is one of the theories for why it's called Shukla). Its Samhita is called the Vajasaneyi Samhita, and its Brahmana is called the Shatapatha Brahmana. The reason the Samhita is called the Vajaseneyi Samhita is because the Shukla Yajur Veda associated with the sage Yajnavalkya, whose surname is Vajasaneya. And the Shatapatha Brahmana gets its name from the fact that it's one hundred chapters long. (Shatapatha means a hundred paths.)
Now most people only know Yajnavalkya as the sage associated with the Brihadaranyaka Upanishad. But actually the Brihadaranyaka Upanishad is part of the Shatapatha Brahmana! In fact, as I discuss in this question, the Shatapatha Brahmana contains within it both an Aranyaka, the Brihad Aranyaka, and an Upanishad, the Brihadaranyaka Upanishad. That's why I said the Shukla Yajur Veda is only a bit cleaner in its division.
As for what it's used for, as I discuss in this answer, a Vedic Yagna involves three priests: a Hotar who chants Rig Veda mantras in their original form heard from the gods, an Adhvaryu who tends to the details of the ritual while chanting ritualistic utterances from the Yajur Veda, and an Udgatri who sings songs from the Sama Veda. (Later a fourth priest, an Atharvana Veda priest called a "Brahmana", was added to the Yagna.) The Shatapatha Brahmana is basically an instruction manual for the Adhvaryu on what he should say and do in the course of a Yagna. (Of course, some Adhvaryus belong to the Krishna Yajur Veda, and thus rely instead on the Brahmana portions found in the Taittiriya Samhita and Taittiriya Brahmana.)
