Fusible web is, very simply, solidified glue that when heated with an iron, will melt and stick (fuse) two surfaces together. Wonder Under, Steam-a-Seam, Misty Fuse and others, are all brand names of different fusible webs. (it's called 'web' because the glue strands are arranged web like)
Fusible interfacing, along with fusible fleece and fusible batting, are all products with a heat sensitive adhesive surface that makes it easier to stick that product to something else.