In Gradle, api and implementation are two different configurations that you can use to declare dependencies in your build.gradle file for a Java or Grails project. They serve different purposes and have different visibility scopes, and understanding when to use each one is important for managing your project's dependencies effectively.

1. api:
   • Use for dependencies that are part of your module's public API.
   • Accessible to other modules that depend on your module.

dependencies {
    api 'com.example:public-library:1.0'
}

2. implementation:
   • Use for internal dependencies not exposed to other modules.
   • Not visible to modules that depend on your module.

dependencies {
    implementation 'com.example:internal-library:1.0'
}

3. Example with both api and implementation:

dependencies {
    api 'com.example:public-library:1.0' // Public API dependency
    implementation 'com.example:internal-library:1.0' // Internal dependency
}

Using these configurations helps manage dependencies' visibility and reduces potential conflicts.

I would suggest to check out The Java Library Plugin documentation.