Heidegger's philosophical magnum opus, written in Germany a few years before the rise of the Nazis. It is easily one of the most divisive tests of the 20th century. Derrida, Foucalt, and nearly all other continental philosophers have been deeply influenced by it and praised it as a masterpiece of modern philosophy while several prominent philosophers have declared that it is nothing but willfully obscure rhetoric and nonsense. Some have even claimed that it is pure jibberish. With that in mind, I'd like to try to write up an amateur summary of Being and Time, at least as I understood the text.
In my opinion the starting point for understanding Being and Time is recognizing that the reading of the text is not linear. There is no sequence of arguments that leads from a clearly defined premise to a clearly defined conclusion. Being and Time explicitly defends the concept of a hermeneutic circle, a process through which one's foreconcepts influence the reading of the text and thus the meaning of the parts depend on the meaning of the whole and the meaning of the whole depends on the meaning of the parts. This approach is based on Heidegger's understanding of time, which is itself the focus of much of the text. Here Heidegger defends the hermeneutic circle approach using the hermeneutic circle approach itself, which makes sense in the context of hermeneutic circle approach. Heidegger has broken down the traditional wall between methodology and conclusion, and with that in mind, the text is very difficult to approach from a linear perspective. Many of the passages only make sense upon multiple readings and may not come into clarity until the entire text is realized.