The Army has more than 200 enlisted jobs to choose from. The Army is unique among all of the active duty services, in that every single enlistment contract includes a guarantee for training in a specific MOS (job). The other services have "guaranteed jobs," as well, but also enlist many recruits into "guaranteed fields," in which they won't find out their specific job until basic training.
The Army is also the only active duty service in which an applicant can "pre-reserve" a job/enlistment program, right in the recruiter's office, prior to going to the Military Entrance Processing Station (MEPS). Army recruiters (including Army Reserves and Army National Guard) have access to a system called called FSR2 or "Future Soldier Remote Reservations System." Any applicant that is a high school senior in good standing or a graduate of high school may pick their job off of the "Request" system from the recruiting station, as long as they "pre-qualify" for enlistment. They then have seven days to go to MEPS, take the physical and then do the final signing for that job. The system also provides shipping date for the job, enlistment bonus/college fund amounts authorized for that job, etc. Of course the job isn't "guaranteed" at this point. The applicant has to meet the qualifications for the job (ASVAB, physical profile, security clearance requirements, ect.), which will be determined during the MEPS trip