Heidegger’s avoidance of traditional philosophical terminology in favour of neologisms derived from colloquial German, most notably Dasein (literally, “being-there”) furthered his goal of dismantling traditional philosophical theories and perspectives.
“Being and Time began with a traditional ontological question, which Heidegger formulated as the Seinsfrage, or the “question of Being.”….”If Being is predicated in manifold meanings, then what is its leading fundamental meaning? What does Being mean?”
If, in other words, there are many kinds of Being, or many senses in which existence may be predicated of a thing, what is the most fundamental kind of Being, the kind that may be predicated of all things?
In order to address this question properly, Heidegger found it necessary to undertake a preliminary phenomenological investigation of the Being of the human individual, which he called Dasein. In this endeavour he ventured onto philosophical ground that was entirely untrodden.
Since at least the time of René Descartes (1596–1650), one of the basic problems of Western philosophy had been to establish a secure foundation for the individual human’s presumed knowledge of the world around him on the basis of phenomena or experiences about which he could be certain (see epistemology). This approach presupposed a conception of the individual as a mere thinking subject (or “thinking substance”) who is radically distinct from the world and therefore cognitively isolated from it. Heidegger stood this approach on its head.
For Heidegger, the very Being of the individual involves engagement with the world. The fundamental character of Dasein is a condition of already “Being-in-the-world”—of already being caught up in, involved with, or committed to other individuals and things. Dasein’s practical involvements and commitments, therefore, are ontologically more basic than the thinking subject and all other Cartesian abstractions. Accordingly, Being and Time gives pride of place to ontological concepts such as “world,” “everydayness,” and “Being-with-others.”
Yet the framework of Being and Time is also suffused by a sensibility—derived from secularized Protestantism—that stresses the paramountcy of original sin. Emotionally laden concepts such as “angst,” “guilt,” and “falling” suggest that worldliness and the human condition in general are essentially a curse. Heidegger, it seems, had implicitly adopted the critique of “mass society” set forth by 19th-century thinkers such as Kierkegaard and Friedrich Nietzsche, a perspective that was well established within Germany’s largely illiberal professoriate in the early 20th century. This theme is illustrated in Being and Time’s treatment of “authenticity,” one of the central concepts of the work.
Heidegger’s view seemed to be that the majority of human beings lead an existence that is inauthentic. Rather than facing up to their own finitude—represented above all by the inevitability of death—they seek distraction and escape in inauthentic modalities such as “curiosity,” “ambiguity,” and “idle talk.” Heidegger characterized such conformity in terms of the notion of the anonymous das Man—“the They.” Conversely, the possibility of authentic Being-in-the-world seemed to portend the emergence of a new spiritual aristocracy. Such individuals would be capable of heeding the “call of conscience” to fulfill their potential for Being-a-self.
Another distinguishing feature of Being and Time is its treatment of temporality (Zeitlichkeit). Heidegger believed that traditional Western ontology from Plato to Immanuel Kant had adopted a static and inadequate understanding of what it means to be human. For the most part, previous thinkers had conceived of the Being of humans in terms of the properties and modalities of “thinghood,” of that which is “present-at-hand.” In Being and Time, Heidegger conversely stressed Being-in-the-world as Existenz—a form of being that is “ecstatically,” rather than passively, oriented toward its own possibilities. From this standpoint one of the distinctive features of inauthentic Dasein is that it fails to actualize its Being. Its existential passivity becomes indistinguishable from the nonecstatic, inert being of things.”
http://www.britannica.com/EBchecked/topic/259513/Martin-Heidegger/284478/Being-and-Time#ref135626
Being Free 