Lieutenant is a junior officer rank in the military. Often, the lieutenant rank is subdivided into grades of seniority.
In the US Army, Lieutenant is an entry level junior officer rank. Within the Lieutenant rank, there are two grades. Second Lieutenant (O-1) is the lowest officer rank and is occupied by newly commissioned officers. Second Lieutenants are entitled to wear a single gold bar on their uniform in accordance with Army Regulations. After a period of time or meritorious action, Second Lieutenants can be promoted to First Lieutenants (O-2). First Lieutenants also wear a single bar, but instead of gold, they wear silver. The next highest rank after Lieutenant is Captain
In the US Navy, Lieutenants are junior officers, but not entry level officers. As in the Army there are two grades of lieutenants. Lieutenant, junior grade (j.g.) (O-2) is the lower ranking officer and Lieutenant (O-3) is the higher ranking officer and equivalent to a captain in the US Army or Air force