What is a Spring Boot starter, and how does spring-boot-dependencies keep all those library versions compatible?

A starter (spring-boot-starter-web, spring-boot-starter-data-jpa, ...) is a pom with no code of its own — just a curated list of dependencies for one job, so you declare one line instead of hunting down a dozen compatible libraries. Version compatibility across all of them comes from spring-boot-dependencies, a Bill of Materials (BOM) imported in dependencyManagement: it pins a tested version for every dependency Spring Boot knows about, so when you bump the Spring Boot parent/BOM version, every managed dependency moves together as a tested set.