Ranks

Joes
E1 - private - PVT
E2 - private - PV2
E3 - private first class - PFC
E4 - specialist - SPC

NCOs
E5 - sergeant - SGT (in charge of a team)
E6 - staff sergeant - SSG (in charge of a squad)
E7 - sergeant first class - SFC - platoon sergeant (PSG) (highest enlisted soldier in a platoon)
E8 - first sergeant (1st SGT) (hightest enlisted soldier in a company)
E9 - command sergeant major (CSM) (highest enlisted soldier in a battalion and up)

Officers
O1 - second lieutenant - 2nd LT - platoon leader (PL - in charge of a platoon)
O2 - first lieutenant - 1st LT - platoon leader (PL - in charge of a platoon)
O3 - captain - CPT - commanding officer (CO - in charge of a company)
O4 - major - MAJ - rarely commands anything, a fairly useless rank :-)
O5 - lieutenant colonel - LTC (in charge of a battalion)
O6 - colonel - COL (in charge of a brigade)
O7 - brigadier general
O8 - major general
O9 - lieutenant general
O10 - general