The United States Army and Marine Corps call their enlisted job specialties, "MOS's," or Military Occupation Specialties.
Bradley Manning was a 35F or 35 Fox, Military Intelligence Analyst, assigned to Company B, 2nd Brigade Special Troops Battalion, 2nd Brigade, 10th Mountain Division Light Infantry (LI), when, the US Government says, he disclosed the largest leak of classified information in US History.
According to the US Army, the mission of the 35F Military Occupational Specialty [MOS] is to "gather, analyze, and report intelligence information that reveals the intended secrets of hostile forces," and 35F "must qualify for a top secret clearance with special access eligibility."
It should be noted, his alleged leak concerned information only marked at the lowest level of classification, "SECRET", or was not classified at all.
For example, on testimony by Army CID Agent, Toni Graham, at Bradley Manning's Article 32 Pretrial Hearing and similarly at the March 16, 2011 Motion Hearing for US v PFC Bradley Manning, there is still uncertainty as to whether or not the July 12, 2007 Baghdad air strike video, commonly known as "Collateral Murder" was classified or not.
35Fs analyze data including weather, terrain, and the position of opposing forces. They are trained to read and interpret maps, electronically plot symbols, and consolidate intelligence data onto a situation map.