There are 3 varients of Kotlin: Kotlin Native, Kotlin JS, and Kotlin JVM (Kotlin JVM communicates perfectly with Java) and Kotlin has these things that Java doesn't:
1. Non null types
2. Better syntax
3. Better null handling (Some things about null in Java that are warnings are hard errors in Kotlin to make sure you handle it)
4. Operator overloading
5. Properties (As opposed to ugly getters and setters)
6. No need for semicolons after every line.
7. The ? operator (While you're doing if (thing != null) thing.function() I'm doing thing?.function())
8. Unsigned types for numbers
and there's more I don't remember off the top of my head.