From Wikipedia: "The tertiary sector of the economy (also known as the service sector or the service industry) is one of the three economic sectors, the others being the secondary sector (approximately the same as manufacturing) and the primary sector (agriculture, fishing, and extraction such as mining)."
The service sector is the part of the economy which, in addition to professional services such as law, medical care, and accounting, which are generally well compensated, consists of providing services to the public such as retail, restaurants, and hotels. It is characterized by low pay and unstable scheduling.
The amount of business, and need for employees, at a retail store or coffee shop such as Starbucks varies widely though out the day, week, month, and season of the year. Weather plays a factor as does the ambient surrounding of the facility. Employee scheduling software is increasingly used to schedule the hours of employees based on need and employee availability. The result is employment of many more part-time workers on changing schedules which often are not available in advance to the worker.