Difference between Isometric, Oblique and Orthographic Drawing
Isometric means "equal measurement". The true dimension of the object is used to construct the drawing. You get the true dimension from either orthographic views or by measuring the object. Because of the convenience of using actual measurements to create the isometric image, it has become the industry standard for parts manuals, technical proposals, patent illustrations and maintainance publications.
The height of the object is measured along vertical lines. The width and depth of the object are measured along the 30 degree to the horizontal plane.
Image from http://www.personal.psu.edu/bjr231/IMG/isometric%20paper%201.GIF
Oblique drawing is the crudest '3D' drawing method but the easiest to master. Oblique is not really a '3D' system but a 2 dimensional view of an object with 'forced depth'.
When using oblique the side of the object you are looking at is drawn in two dimensions, i.e. flat. The other sides are drawn in at 45 degrees but instead of drawing the sides full size they are only drawn with half the depth creating 'forced depth' adding an element of realism to the object.
Even with this 'forced depth', oblique drawings look very unconvincing to the eye. For this reason oblique is rarely used by professional designer and engineers.
Orthographic Projection is a way of drawing an 3D object from different directions. Usually a front, side and plan view are drawn so that a person looking at the drawing can see all the important sides. Orthographic drawings are useful especially when a design has been developed to a stage whereby it is almost ready to manufacture.