What is a Spring Boot 'starter', and why does adding one dependency configure so much automatically?

A starter (e.g. `spring-boot-starter-data-jpa`) is just a Maven/Gradle dependency descriptor that transitively pulls in every library that feature area needs (Hibernate, the JPA API, a connection pool) plus nothing else — it doesn't contain code itself. The 'automatic' part comes from auto-configuration reacting to those libraries now being on the classpath, not from the starter doing anything special.