Inversion of control (IoC) is an object oriented programming technique in which object coupling is bound at run time by an assembler object and is typically not known at compile time. This binding process is usually achieved through dependency injection. You can achieve this by designing to interfaces. Read more @ www.javajee.com/inversion-of-control-and-dependency-injection.

You can download the Spring Tool Suite or the Spring Framework jars alone or use the maven/gradle repositories.

Using Spring Boot can make the getting started part much easier. However, a proper understanding of the non Spring Boot way of doing things will help you understand Sping Boot better.

Inversion of Control is a pattern where the controls of dependencies are moved out usually from program components to container. The usual control flow of a program is inverted, and hence it is called so. 

For instance, instead of using a new keyword to create an object and setting dependencies, a dependency injection framework (e.g. spring container) will create the object and its dependencies for you at runtime and inject them into your code.