Apps Script services provide ways for your script to easily access data on
Google and external systems. These services are built into the Apps Scipt
environment so you don't have to import them or implement authorization
controls yourself. Services are expressed as global objects with associated
Apps Script services are divided into three categories:
G Suite services are services that let you access the data of G Suite products such as Drive, Gmail, Calendar, Docs, Sheets, and others. These services are separate from APIs created for those products; in general, G Suite services are easier to use than G Suite APIs, but are optimized for only the most common use cases.
Advanced Google services provide the means to use Google APIs (including but not limited to the G Suite product APIs) within Apps Script. Advanced Google services are really just thin wrappers around those APIs, and are not an API themselves. For more details, see Advanced Google services.
Script services are utility services that are not connected to a particular G Suite product. They let you do things like log information, create HTML, compress data, and more.