Poststructuralism is a set of theories and ideas that describe how human beings relate through language and meaning to the world and themselves. There is not so much a theory as a set of theorists writing in the 1960s in France - notably Roland Barthes, Michel Foucault, Jacques Derrrida, Julia Kristeva and Gilles Deleuze. These writers acted in reaction to the structuralism put forward by linguist Ferdinand de Saussure and the anthropologist Claude Lévi-Strauss.
Poststructuralists are critical of the idea of the self, seeing human beings as being made up of competing identities and facets. The meaning of texts given by authors becomes less important than the view given by the reader - literary texts, under a poststructuralist view, have multiple, often contradictory, meanings and purposes contained within. The relative lack of importance placed on the views of the author is embodied in "The Death of the Author", an essay by Roland Barthes.
Poststructuralism is often understood as being a step on the way to postmodernism - one of the key writers on postmodernism, Jean-François Lyotard, was a poststructuralist at one point - although there are many poststructuralists who did not go on to identify as postmodernists.