The expression semikha comes from the Hebrew word l'smoch, meaning "to lean on." The word appears in Joshua's succession as leader of the Children of Israel after the death of Moses (Numbers 27:18-23). Moses lays his hands on Joshua, signifying the change in leadership. One who is ordained is called a mus-makh.
Today, the practice of rabbinic ordination does not include any "laying of the hands," but a written document. The document gives rabbis the authority to answer questions of Jewish law and become religious community leaders.
Most of those who receive semikha do so from the Orthodox yeshivot. These rabbinic students often pursue other professional fields as well, and gain rabbinic ordination to show that they are learned in matters of halacha, or Jewish law. Those granted semikha from Conservative or Reform movements often teach or become Rabbis.
The Reconstructionist, Reform, and Conservative movements all ordain women for the Rabbinate. Blu Greenberg is a liberal orthodox scholar who contends that in the future Orthodoxy will start to ordain women. Most of mainstream Orthodoxy does not share her sentiments.
Sources: Telushkin, Joseph. Jewish Literacy: The Most Important Things to Know About the Jewish Religion, Its People and Its History. NY: William Morrow and Co., 1991 and Judaism 101.