Groovy is a dynamically-typed scripting language. Code written in the Java language is valid in Groovy, so users not familiar with Groovy may resort to the Java syntax. All transform functions available for you in Studio are written in Groovy.
Groovy was chosen as the basis for the Transform API because it is flexible and easy to use. Additionally, although it is a dynamic language, it can use static compilation and static type checking to make it less error-prone at runtime.
Studio lets you use many features of the Groovy language when writing your own custom transformations; it does, however, impose a few restrictions for security reasons. For more information, see Unsupported Groovy language features.
You can find more information on Groovy, including tutorials, in the Groovy documentation.