I'll use very small words. Do you know what an immigrant is?
FYI
http://education.yahoo.com/reference/dictionary/entry/immigrantim·mi·grant (m-grnt) KEY
NOUN:
A person who leaves one country to settle permanently in another.
A plant or animal that establishes itself in an area where it previously did not exist.
ADJECTIVE:
Of or relating to immigrants or the act of immigrating.
http://education.yahoo.com/reference/dictionary/entry/illegal;_ylt=AvoWFuo2qgGyjA6ZhkrKuS.sgMMFil·le·gal (-lgl) KEY
ADJECTIVE:
Prohibited by law. Prohibited by official rules: an illegal pass in football.
Unacceptable to or not performable by a computer: an illegal operation.
NOUN:
An illegal immigrant.
http://education.yahoo.com/reference/dictionary/search;_ylt=AjMh.Kldi3Y60Gp5Eq9lEO2qgMMF?p=aliena·li·en (l-n, lyn) KEY
ADJECTIVE:
Owing political allegiance to another country or government; foreign: alien residents.
Belonging to, characteristic of, or constituting another and very different place, society, or person; strange. See Synonyms at foreign.
Dissimilar, inconsistent, or opposed, as in nature: emotions alien to her temperament.
NOUN:
An unnaturalized foreign resident of a country. Also called noncitizen .
A person from another and very different family, people, or place.
A person who is not included in a group; an outsider.