Scala is an object-oriented and functional programming language developed at the epfl and running on top of the jvm (java virtual machine).
It brings powerful features to the Java ecosystem, such as a unified type system, a very well designed data structure library, concurrent actor model and futures, pattern matching and mixin-based composition.
In short it leverages all the existing possibilities in Java and extends it further. The result is a more flexible and reusable code base written with less effort.