External factors
It is the external environment of an enterprise which includes: Social/ cultural environment, technological, political, legal, economical environment.

Internal factors
Porter's Five Forces include three forces from 'horizontal' competition: threat of substitute products, the threat of established rivals, and the threat of new entrants; and two forces from 'vertical' competition: the bargaining power of suppliers, bargaining power of customers. Other internal factors are: financial, human, technological resources, sales force, raw material, etc.
Source: http://en.wikipedia.org/wiki/Positioning_(marketing) 

In marketing, positioning has come to mean the process by which marketers try to create an image or identity in the minds of their target market for its product, brand, or organisation. It is the 'relative competitive comparison' their product occupies in a given market as perceived by the target market.
Source: http://en.wikipedia.org/wiki/Positioning_(marketing)