To all:
For those who have trouble understanding basic existential concepts, may I suggest William Barrett's work _Irrational Man_. On pp. 218-219 of this monograph, Barrett succinctly explains the significance of Dasein in Heideggerian thought. Dasein (Heidegger claims) points to human Being-in-the-world. Alternatively, we could also say that Dasein (literally, Being-there in German) is the human field(region) of Being.
Heidegger employs the term Dasein to avoid making one very critical theoretical mistake. Rene Descartes had previously posited a dualistic form of philosophical anthropology. He had thought of man in terms of the res cogitans and the res extensa. Descartes, in other words, had formulated a dualistic theory of man. He thus made it possible for man to be an object or a subject. This French thinker also believed there was a radical difference between thinking substance (res cogitans) and extended substance (res extensa). Philosophers still have a difficult time trying to "unite" the two substances and figure out how they relate to one another.
By starting with the cogito, Descartes also implied that man could BE without Being-in-the-world. Heidegger rejected such a dualistic, objectifying, and isolating notion of man. As Barrett words matters: "One of the most remarkable things about Heidegger's description of human existence is that it is made without his using the term 'man' at all! He thereby avoids the assumption that we are dealing with a definite object with a fixed nature--that we already know, in short, what man is."
By employing the term Dasein, Heidegger also makes it clear that human existence is not confined to the "envelope" of the skin. We are are, says Heidegger, when we are in-the-world.
Duns the Scot