The introduction starts by asking what is a woman? and comes to the conclusion that women are primarily defined negatively, as not-men:
La femme a des ovaires, un utérus; voilà des conditions singulières qui
l'enferment dans sa subjectivité; on dit volontiers qu'elle pense avec ses
glandes. L'homme oublie superbement que son anatomie comporte aussi des
hormones, des testicules.
and she quotes Aristotle: «La femelle est femelle en vertu d'un certain manque de qualités». So then she talks about the concept of the other and the way in which humans find Otherness in different groups - les Juifs, les Noirs. But women aren't like ethnic groups - Jews aren't Others to other Jews (or at least, not because of their Jewishness). Women live alongside men, in equal numbers to men, but are still seen as Other by those men. In this regard, she compares women with the proletariat: a group which exists alongside the bourgeoisie, and have existed as long as they have.
If you want to read her in English, there's an english translation of Le deuxième sexe at the Marxist Internet Archive. I'm reading her in french because, well, why not stretch my brain for once. Also I read somewhere that the translation doesn't do her justice.