Projects in IntelliJ IDEA allow you to conveniently manage modules and share settings between them. These settings include an SDK, a language level, and a compiler output.
Modules are independent entities. Each module can have its own settings and can be configured to develop framework-specific applications. Modules can reference other modules, i.e. depend on each other.
In IntelliJ IDEA, you can integrate projects with build tools, such as Maven or Gradle. In this case, the IDE will provide you with coding assistance for editing the build file, code completion, inspections and quick-fixes, refactorings, and so on.