Lieutenant

A Lieutenant is an officers rank in the military, police and fire departments. It is an entry level supervisory position. The word comes from the French language. Lieu means "place" and tenant means "holding". A lieutenant is someone who takes the place of a superior when that person is not available. It can also mean a deputy position to assist someone else to carry out their job. 
Military.
In many military organizations, a lieutenant is an entry level commissioned officer's rank. Generally, but not always, it is divided into two levels. 
Second Lieutenant.
A second lieutanant (2LT) (2ndLt) (2d Lt) (O1) is the lower rank of lieutanant. A second lieutenant is addressed as "lieutenant" the same as a first lieutenant. In the United States Army, for example, a second lieutenant leads a platoon made up of two or more squads or about 16 to 44 soldiers. In this capacity he or she functions as a platoon leader or commander. The length of time an officer can hold the rank of second lieutenant is different in different services. For example, in the British Army it may be held for up to 2 years.
First Lieutenant.
A first lieutenant (1LT) (1stLt) (1st Lt) (O2) is the senior rank of lieutenant. A first lieutenant is also addressed as "lieutenant". In the army, a first lieutenant often functions as the executive officer of a Company (military unit). A company may have from 110 to 140 personnel. In the British Army, an officer may hold this rank for up to three years.
There is great variation in the insignia used worldwide. In most English-speaking and Arabic-speaking countries, as well as a number of European and South American nations, full lieutenants (and equivalents) usually wear two stars (pips) and second lieutenants (and equivalents) one. An example of an exception is the United States, whose armed forces distinguish their lieutenant ranks with one silver bar for first lieutenant and one gold (brass) bar for second lieutenant.
In many Navies including the United States Navy, a lieutenant is the equivalent rank to an army captain and a lieutenant (junior grade) is the same as an army first lieutenant. The equivalent to a second lieutenant is the naval rank of ensign.