Apprenticeships form a traditional method of training a new generation of skilled crafts practitioners. Apprentices (popularly "prentices") built their careers from apprenticeships.
The system of apprenticeship dates at least from the days of guilds. A boy (in those male-centric medieval days) would join the household of a master craftsman, perform menial tasks and learn the trade. Eventually he might reach a skill level where the relevant guild would recognise him as a journeyman. Ultimately, after years of achievement, he could become a master himself. (Marrying his own master's daughter could aid in this process.)
Subsequently governmental regulation and the licensing of polytechnics and their ilk formalised and bureaucratised the details of apprenticeship, which still survives in attenuated form.
Universities still echo apprenticeship schemes in their production of scholars: bachelors are promoted to masters and then produce a thesis under the oversight of a supervisor before the corporate body of the university recognises the reaching of the standard of a doctorate.