OUGD404 Double Page Spread: Italics and Obliques

In typography, italic type is a cursive typeface based on a stylized form of calligraphic handwriting. Owing to the influence from calligraphy, such typefaces often slant slightly to the right. Different glyph shapes from Roman type are also usually used—another influence from calligraphy. True italics are therefore distinct from oblique type, in which the font is merely distorted into a slanted orientation. However, upper-case letters are often oblique type or swash capitals rather than true italics.

This style is called "italic" for historical reasons. Calligraphic typefaces started to be designed in Italy, for chancery purposes. Ludovico Arrighi and Aldus Manutius (both between the 15th and 16th centuries) were the main type designers involved in this process at the time.aa

Italics are drawn out separately from the regular letterforms in a typeface based on an axis ranging from 7-20 degrees. Italics have a calligraphic style and can sit close due to the angle they have individually drawn at.

aa

Obliques were created because italics were considered inappropriate for the industrial and non-calligraphic designs of most sans-serif typefaces. An oblique is essentially a copy of the roman character put at an angle often around 8-12 degrees. 

Italic is a special version of the font, whereas an oblique version is just the regular version inclined a bit. So both are slanted and related to the regular font, but italic will have special letterforms made especially for it.

An italic is created by the type designer with specific characters (notably lowercase a) drawn differently to create a more calligraphic, as well as slanted version.

Source: http://stackoverflow.com/questions/1680624/css-font-style-italic-vs-oblique