Google just announced that it will officially support Kotlin on Android as a “first-class” language. Kotlin is a super new programming language built by JetBrains, which also coincidentally develops the JetBrains IDE that Android Studio — Google's official developer tool — is based on. Like Java, which is the default language for Android development, Kotlin is a language that runs on the JVM (Java Virtual Machine), and it's already possible to use Kotlin and many other JVM languages for Android development. Android doesn’t use the JVM exactly, but the Java roots are strong, and Kotlin’s interoperability with Java have made it a popular choice for developers. Official Google support will be a huge boost for the nascent language, however, and will presumably make working with Kotlin on Android a lot more natural.
Kotlin tools will be included with Android Studio 3.0 by default, and JetBrains and Google are pledging to support the language going forward.
Kotlin as a language has a lot of similarities to Java in structure — it's object oriented and statically typed, and designed for similar problems Java solves. But because it's a clean slate in many ways, Kotlin adds a lot of nice-to-have features, a much cleaner syntax, ideas from functional programming, and other enhancements over Java.