Short answers, pulled from the story.
Being and Time, published in 1927, attempts to revive ontology through an analysis of Dasein, or being-in-the-world. It rejects Descartes' picture of the human being as a detached observer of objects, arguing instead that subject and world are inseparable. The book also provides an extended treatment of authenticity as a means to grasp and confront the finite possibilities of the individual.
Being and Time is regarded as the most influential version of existential philosophy and had a decisive impact on French philosophy, psychoanalysis, literary theory, and continental philosophy broadly. Simon Critchley wrote in 2009 that developments in continental philosophy after Heidegger cannot be understood without understanding Being and Time. Jean-Paul Sartre's existentialism of 1943 has been described as merely a version of it.
Dasein is a German term meaning literally being-there, coined by Heidegger to describe the living being through its activity of being in the world. It rejects the Cartesian split between a subjective mind and an objective world, treating being-in-the-world as an essential, unitary characteristic rather than an accidental combination of parts.
Almost all of the central concepts in Being and Time derive, according to Christian Lotz, from Augustine, Luther, and Kierkegaard. Edmund Husserl, Heidegger's mentor, provided the phenomenological method that the book adopts and modifies. Heidegger also explicitly credited Wilhelm Dilthey within the text for the hermeneutic circle and theories of history.
Being and Time was published in 1927 with a dedication to Edmund Husserl, who had championed Heidegger's work and helped him secure the chair in philosophy at the University of Freiburg in 1928. In 1941, Heidegger, then a member of the Nazi Party, agreed to remove the dedication because Husserl was Jewish. The dedication was restored in the 1953 edition.
Heidegger originally planned a second volume critiquing Western philosophy but quickly abandoned the project. He later identified his 1935 lecture course Einfuhrung in die Metaphysik as relevant to those concerns, and his 1962 lecture Zeit und Sein at the University of Freiburg was described as his most direct confrontation with the themes of Being and Time.