The terms locator and geocoding service are sometimes used synonymously, but there is an important difference between the two.
A locator is the developer's term for an object that converts textual or numeric descriptions of locations to geometric representations of those locations. A geocoding service is the user's term for a particular type of locator that creates geometry for addresses. Developers refer to geocoding services as address locators.
While the term geocoding has traditionally been used to describe the process of converting addresses to geometric representations, it is not general enough to describe the developer experience in ArcGIS. ArcObjects includes a framework for developing and using locators that convert any manner of textual or numeric descriptions of locations to geometric representations. Address locators use this framework to provide geocoding functionality in ArcGIS.
Just as locators are to geocoding services, so are locator styles to geocoding service styles. A locator style is the developer's term for a template on which new locators are based, while a geocoding service style is the user's term for a template on which new geocoding services are based.