Isometric 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.

Source:http://www.foothillsgraphics.com/iso.htm

Oblique Drawing

Oblique drawing is drawing a 3-d image with one side of the object with its true shape and the rest of the sides with different shapes.Eg: cube, cuboid

Source: Wiki answers

Orthographic Drawing

Orthographic drawing is drawing a 3d image from different 2d views. This enables us to see the 3d image from different sides.

Source:http://www.cdli.ca/depted/g7/ortho.htm