In orthographic drawing the three "dimensions" of an object are depicted in three views, each showing the object from one of three perpendicular planes.
An Isometric drawing is a quasi 3d drawing that shows the height width and depth of the object in a single view where the viewpoint is at a 45 degree angle from each of the perpendicular planes of the orthographic view.
Isometric differs from a perspective view in that all lengths are shown true length. In a perspective drawing the objects that are depicted to be more distant are shortened to make the view more realistic looking.