Please define postmodernism in simple and easy terms.
This is a great question and I completely agree with you. Too often people give very difficult definitions of postmodernism. In light of this, let me offer you a few points that will help you understand postmodernism.
First, postmodernism does not believe in a totalizing grand narrative. In other words, postmodernism resists the idea that there is a dominant perspective. M. Foucault, one of the great voices of postmodernism, speaks of the archaeology of knowledge. By this he means that there are many different perspectives that are valid.
Second, in light of this, there is a hermeneutic of suspicion. This means that there is a suspicion when it comes to anything that claims to be objective. Postmodernism realizes that all people are coming from a point of view, based on their experiences and historical context. Here is a great quote by Michel Foucault:
"Each society has its régime of truth, its ‘general politics’ of truth: that is, the types of discourses which it accepts and makes function as truth; the mechanisms and instances which enable one to distinguish true and false statements, the means by which each is sanctioned; the techniques and procedures accorded value in the acquisition of truth; the status of those who are charged with saying what counts as true."
Third, there is an emphasis that all knowledge is relative. These three points should get you started.
Please allow me to add: postmodernism is misnamed, because it sounds like it came after Modernism. Actually it is a part of Modernism, that part dealing with form rather than content. Modernism concerns itself with the real, pedestrian world, not with ancient kings, warriors, etc. and not with imaginary figures, fairies, ghosts, and the like, but with everyday people and their dilemmas. Authors found that the traditional ways of telling stories--omniscient narrators, narratives, plots with structure, etc., had limitations, so they began experimenting with such structural devices as unreliable narrators, linguistic variation, nonlinear plots, etc., and that is called "postmodernism" in that it emerged after the impulse toward modern subect matter.