A service business does not sell products, but instead sells some kind of service to its customers. Service businesses might be aimed at selling services to consumers or they might be aimed at selling services to other businesses.
A service business might also exist within a larger business structure, as it is possible for a business structure to be primarily focused on selling products, while then having a service business branch within the overall structure to help provide some kind of service associated with the products being sold. For example, a computer business might have a service business branch which acts as a technical support service for customers.
Service businesses, of necessity, have a relatively different set of principles and requirements than do product-based businesses. A service business, for example, would not have a clear marketable product to display to potential buyers. Instead, a service business would likely have to display good reviews from prior clients, as well as a satisfactory customer service element of the business to ensure that customers are happy with the entirety of the service business.
Service businesses still form a very significant part of the overall economy, when one considers the sheer variety of businesses which fall under the overall purview of service businesses. Lawyers, for example, qualify as one kind of service business in that they provide a service to their clients without producing an actual clear cut physical product.
Similarly, those who might market any special skills they have to students, such as music teachers, are inherently members of a service business.