The Coast Guard is the only service that doesn't automatically send most recruits straight to job training after basic training. Instead, following basic training leave, most new recruits are sent directly to their first duty assignment.
At your first duty assignment, you'll be classified as an undesignated seaman (SN), or fireman (FN), which means you don't have a specific rating (job) and are available to do anything number of the jobs which need to be done on the ship or base. These tasks include general ships work, such as painting, general machine maintenance and operations, scullery duty (working in the ships galley), performing maintenance on the ships safety systems, helping fueling operations, etc. Think of it as an apprenticeship of sorts.
After completing your seaman or firemen training and time-in-service requirements, you will be given the chance to strike (on-the-job-training) for a specific rating (Coast Guard job) or apply for more formal rate training at a training center, known as “A” school. If you apply for one of the specific Coast Guard ratings that require more formal training, you will be assigned to an “A” school.
NOTE: If you wish to work on Coast Guard aircraft and related systems you must first be assigned to an air station as an airman (AN) to complete your basic aviation training requirements before heading off to “A” school.
Once striker training or “A” school training is completed you are given a designator and in most cases you will be promoted to 3rd Class Petty Officer.
This system has its advantages. In the other branches, you may be required to select your job in advance, based on what you've read about it in an official military recruiting job description. These job descriptions are designed to make any job sound interesting and exciting. In the Coast Guard, you get a couple of years to actually scope out the various jobs before making any choices.