From the wiki page: (ref: http://en.wikipedia.org/wiki/Mara_(demon
"In traditional Buddhism four senses of the word "mara" are given:
1.Klesa-mara, or Mara as the embodiment of all unskillful emotions.
2.Mrtyu-mara, or Mara as death, in the sense of the ceaseless round of birth and death.
3.Skandha-mara, or Mara as metaphor for the entirety of conditioned existence.
4.Devaputra-mara, or Mara the son of a deva (god), that is, Mara as an objectively existent being rather than as a metaphor.
Early Buddhism acknowledged both a literal and "psychological" interpretation of Mara. Mara is described both as an entity having a literal existence, just as the various deities of the Vedic pantheon are shown existing around the Buddha, and also is described as a primarily psychological force - a metaphor for various processes of doubt and temptation that obstruct spiritual practice."