What are `spring-boot-starter` jars?

Those dependencies are meant to provide a unified entry to an ad-hoc skeleton project with all needed dependencies.

They should usually be inherited from your project descriptor (pom.xml) so that you get all parent dependecies with configured versions. No more burden to be done on the developer side:

Starter POMs are a set of convenient dependency descriptors that you can include in your application. You get a one-stop-shop for all the Spring and related technology that you need, without having to hunt through sample code and copy paste loads of dependency descriptors. For example, if you want to get started using Spring and JPA for database access, just include the spring-boot-starter-data-jpa dependency in your project, and you are good to go.

Reference, Spring Boot Starter POM.

Edit:

Those POMs can be used to synthesize the dependencies that can be used for a certain kind of project, e.g. for a simple Spring MVC project, the following artifacts are to be included (Read from spring-boot-starter, spring-boot-starter-web, spring-boot-starter-security respectively):

• Spring Boot artifacts:
  • org.springframework.boot:spring-boot
  • org.springframework.boot:spring-boot-autoconfigure
  • org.springframework.boot:spring-boot-starter-logging
• Spring Core, Web, MVC, Security artifacts:
  • org.springframework:spring-core
  • org.springframework:spring-web
  • org.springframework:spring-webmvc
  • org.springframework:spring-beans
  • org.springframework:spring-context
  • org.springframework:spring-expression
  • org.springframework:spring-aop
  • org.springframework.security:spring-security-config
  • org.springframework.security:spring-security-web
  • org.hibernate:hibernate-validator
  • com.fasterxml.jackson.core:jackson-databind

The artifacts can be found seamlessly navigating search results in the maven central repository.

Note that this may not be a full fledged reference list as some components may be mising thus the artifacts are subject to be updated.

Simply stated they are dependency descriptors that list transitive dependencies with versions that are tested to work together to save you time from trying to put libraries together that take care of some facet of an application.