It depends on the context - it can refer to culture, art, history, literature, architecture. It's a movement that was a reaction to 'modernism' - and came 'after'.
In art and culture it is basically about moving away from originality and authenticity and questioning whether this has ever really existed. It's moving towards multiplicity of meanings, towards quotation, referencing - imagine a collage of lots of things brought together. In art and culture it's seen as subversive and often humorous - although it has a whole different set of values when talking about post modern philosophy, ethics or epistemology - so really does depend on context.
In TV, those shows like the 100 greatest blah blah - that's postmodern.
In architecture something like the Pompidou Centre would be postmodern because traditionally architecture was about taking something functional and trying to make it attractive, whereas with the postmodern it's exposing the bits that are usually hidden, or sticking greek columns on a sky scraper. It's about jokes in a way.
I'm rambling and probably making no sense.