Isometric, Oblique and Orthographic

Isometric: It is a form of representing three-dimensional objects in two dimensions. It is the easiest way of drawing 3-D. It is a form of graphical projection.

Oblique: It is a type of parallel projection. It is easer to master but the crudest form of drawing in 3-D. It uses forced depth to make a two-dimentional object look like a three-dimensional object.

Orthographic: It is also a form of parallel projection but the view direction is orthogonal(perpendicular) to the projection plane. It is a way of drawing physical object with details and is mostly used in designing houses.

Sources: http://en.wikipedia.org/wiki/Isometric_projection

http://en.wikipedia.org/wiki/Oblique_projection

http://en.wikipedia.org/wiki/Orthographic_projection

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