Spring Boot
Spring Boot is an opinionated, and ready to get-started framework built on top of Spring framework. It helps in creating stand alone and production ready application very quickly with minimal configuation efforts.
What is opinionated way of spring boot
Opinionated way means spring boot follows an approach which guides to develop applications in their own way. Spring boot has its own way of creating every type of applications. It has a set design pattern for various types of applications. The develpers only needs to plugin their logic. The framework will take care of rest of the configurations. These design patterns are known as starter kits in spring boot. All these starter kits come as maven/gradle dependencies.
For an example, if you want to create a web application, there is a spring-web-starter kit. You only need to have that starter kit in your pom.xml file.
What is need of spring boot when spring is already there
We know that it is the era of microservices. Earlier, when monolythic application were developed, spring was good enough for the development. But with advent of microservices, it was necessary that the development and deployment of services was much faster than usual. There came the need of spring boot. With spring boot, you can rapidly develop microservices, test them and deploy them.