The quick way to visualize this is to compare isometric and perspective drawings of a cube. In the isometric drawing, all the edges of the cube would be parallel in sets of four. In the perspective drawing, the edges would taper towards one or more vanishing points.
The isometric drawing, being easier to construct, perserving all scales and dimensions, is the preferred method for mechanical drawings, and are practical for use in the shop. The perspective drawing, which are trickier to draw properly, and does not preserve scales and dimensions, is the preferred method for architectural drawings, because they illustrate what the eye actually sees.
