"Juxtaposition" = "putting something next to something else".
In some modernist texts, for instance, the writer may present two images, side by side ("juxtapose" them). The connection between the images is not necessarily made explicit: the reader is required to draw his own conclusions.
In this poem, for instance, Pound juxtaposes two images, faces in a crowd and petals:
The apparition of these faces in the crowd:
Petals, on a wet, black bough.
No explicit connection is made; the reader must infer that the faces are like petals.
"Intertextuality" can mean two things: a) deliberate or unconscious allusions in one text to another text, or b) the necessary interdependency of all literary texts.