The Strait of Hormuz is the entrance into the
Persian Gulf from the
Gulf of Oman/
Arabian Sea/
Indian Ocean. The coast of
Iran to the north takes a little dip inward, and to the south a small
peninsula sticks up from the body of the
Arabian Peninsula with parts of the countries of
Oman and the
United Arab Emirates on it, so that ships have to go north and then turn rather sharply west-southwest to get between the peninsula and the Iranian
mainland, as well as avoiding numerous islands mostly belonging to Iran. During the
Gulf War, the keeping of the Strait of Hormuz open for anti-Iraq forces was of great importance, since it is the only sea route to
Kuwait.
There is a nice map of the strait at http://gulf2000.columbia.edu/reference/hormuz.html