Semitic is an adjective which in common parlance mistakenly refers specifically to Jewish things, while the term actually refers to things originating among speakers of Semitic languages or people descended from them, and in a linguistic context to the northeastern subfamily of Afro-Asiatic.
In a linguistic context, it refers to speakers of a subgroup of the Afroasiatic languages including, among others, Arabic, Hebrew, Canaanite, Akkadian, and Amharic. These peoples were all descendants of the Phoenicians, which was the Greek name for the Canaanites. The Semitic languages historically stretch all the way along the southern Mediterranean Sea to the Atlantic Ocean, into Mali and along the coast of the Red Sea all the way to Somalia in Africa. Semitic languages are also spoken in European Malta and on Socotra in the Indian Ocean. Additionally, millions of Muslims speak Classical (Qur’ānic) Arabic as a second language, and many Jews all over the world speak Hebrew as a second language. It should be noted that Coptic, Berber, Somali, and many other related Afro-Asiatic languages within this area do not belong to the Semitic subgroup.
In a religious context, it refers to the religions associated with the speakers of these languages: thus Judaism, Christianity, and Islam are often described as "Semitic religions". This term can equally include the polytheistic religions (such as the religions of Tammuz or Adad) that flourished in the Middle East before the Abrahamic religions.
Outside linguistics, the term's primary use nowadays is to refer to the ethnic groups who have historically spoken Semitic languages. The best way known to test an ethnic group's common physical descent is through genetic research. Though in genetic research no significant common mitochondrial results have been yielded, genetic Y-chromosome links between Near-Eastern peoples like the Palestinians, Syrians and ethnic Jews have proved fruitful (see Y-chromosomal Aaron). While population genetics is still a young science, it seems to indicate that a significant proportion of these peoples' ancestry comes from a common Near-Eastern population to which (despite the differences with the Biblical genealogy) the term Semitic has been applied (http://foundationstone.com.au/HtmlSupport/WebPage/semiticGenetics.html).
Anti-Semitism is a term whose most common usage typically is to describe anti-Jewish statements or beliefs. However, it is increasingly used by people who apply the word in reference to any Semitic people, especially as a reference to anti-Arabism.
In Genesis Shem is described as the father of the Assyrians, Chaldeans, Aramaeans, Sabaeans, and Hebrews, all of whose languages are closely related; the linguistic family containing them was therefore named Semitic. The Canaanites and Amorites spoke a language belonging to this family, and are therefore also termed Semitic in linguistics despite being described in Genesis as sons of Ham, one of Noah's sons. (See: Sons of Noah). Shem is also described in Genesis as the father of the Elamites and the far-eastern descendants of Lud, whose languages were not Semitic. As this list makes clear, its meaning has shifted considerably.