The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, 4th edition ("DSM-IV") includes in its description of a phobia an "intense anxiety" which follows exposure to the "object of the phobia, either in real life or via imagination or video..." For xenophobia there are two main objects of the phobia. The first is a population group present within a society, which is not considered part of that society. Often they are recent immigrants, but xenophobia may be directed against a group which has been present for centuries. This form of xenophobia can elicit or facilitate hostile and violent reactions, such as mass expulsion of immigrants, or in the worst case, genocide.
The second form of xenophobia is primarily cultural, and the objects of the phobia are cultural elements which are considered alien. All cultures are subject to external influences, but cultural xenophobia is often narrowly directed, for instance at foreign loan words in a national language. It rarely leads to aggression against persons, but can result in political campaigns for cultural or linguistic purification. Isolationism, a general aversion of foreign affairs, is not accurately described as xenophobia. Additionally, in the world of science fiction, xenophobia usually refers to a fear or hatred of extraterrestrial cultures or beings.
http://en.wikipedia.org/wiki/Xenophobia