PRIVATE FIRST CLASS (PFC)
(Addressed as "Private")
PV2s are promoted to this level after one year—or earlier by request of supervisor. Individual can begin BCT at this level with experience or prior military training. Carries out orders issued to them to the best of his/her ability.
SPECIALIST (SPC)
(Addressed as "Specialist")
Can manage other enlisted Soldiers of lower rank. Has served a minimum of two years and attended a specific training class to earn this promotion. People enlisting with a four year college degree can enter BCT as a Specialist.
CORPORAL (CPL)
(Addressed as "Corporal")
The base of the Non-Commissioned Officer (NCO) ranks, CPLs serve as team leader of the smallest Army units. Like SGTs, they are responsible for individual training, personal appearance and cleanliness of Soldiers.
SERGEANT (SGT)
(Addressed as "Sergeant")
Typically commands a squad (9 to 10 Soldiers). Considered to have the greatest impact on Soldiers because SGTs oversee them in their daily tasks. In short, SGTs set an example and the standard for Privates to look up to, and live up to.
Leadership responsibility significantly increases in the mid-level enlisted ranks. This responsibility is given formal recognition by use of the terms noncommissioned officer and petty officer. An Army sergeant, an Air Force staff sergeant, and a
Marine corporal are considered NCO ranks. The Navy NCO equivalent, petty officer, is achieved at the rank of petty officer third class.
http://www.defense.gov/about/insignias/enlisted.aspx