S1 is administration, basically this shop handles personnel stuff, from reassignments within a given unit to losing your 4187 for the 15th time in a month, processing awards, and UCMJ matters
S2 deals with military intelligence, they typically handle priority intel requirements collection, collection management, enemy I&W, security clearance records and classified information control. SOI is also a function of the S2 shop
S3 deals with training planning and coordination in garrison and while deployed makes operational plans and advises the commander on how best to execute a given collective task. Is also usually responsible for ammunition.
S4 – supply, handles equipment distribution and replacement for a given unit, deals with field loss/combat loss of equipment, has to keep track of the endless property books
S5 is actually rolled into the S3 shop in the US Army
S6 – Handles communications such as Nodal Networks, Tropo, TACSAT, and other high end communications goodies, when the stuff works.
G Acronyms – These are given to designate staff, not department.
G1 Assistant Chief of Staff, G1 (Personnel)
G2 or G-2 (1) Assistant Chief of Staff, G2 (Intelligence); (2) Army or Ground Directorate of Intelligence;
(3) General Corps Staff Intelligence Officer
G3 Assistant Chief of Staff, G3 (Operations and Plans)
G4 Assistant Chief of Staff, G4 (Logistics)
G5 Assistant Chief of Staff, G5 (Civil Affairs)
G6 Director(ate) of C4
G8 Director(ate) of Resource Management