Most interpretations of the photo suggest that the feature is a natural landform, one of many mesas that scatter Cydonia. In this view of things, the appearance of a face is given by a combination of the angle of the lighting (with the sun low on the Martian horizon at the time the photo was taken), the low resolution of the photo tending to smooth out the irregularities of the surface, and the human brainís tendency to recognise familiar patterns, especially faces (pareidolia). Finally, a gap in the data sent back by Viking 1 created a black spot exactly where a nostril would be located on a human face. Many other such spots are visible in the photo.
This interpretation is supported by later photographic evidence from the Mars Global Surveyor probe in 1998 and 2001 and the Mars Odyssey probe in 2002. Photographed under completely different lighting and at much higher resolution, the feature looks very little like a face:
Another interpretation of the photo is that it represents an artificial monument of some kind, and some have claimed that its existence is proof that intelligent extraterrestrials inhabited or visited Mars at some point in the distant past. The most notable advocate of this theory is Richard Hoagland. In his 1987 book The Monuments of Mars: A City on the Edge of Forever, Hoagland interpreted other nearby surface features as remnants of a ruined city and artificially-constructed pyramids. The publication of this book has done much to encourage and popularise belief in the artificial nature of the face.
See also: Life on Mars