If you don't already know Java, these pages are for you.
The Groovy Math facilities in depth:
Groovy Integer Math - choose from many types of integers
Groovy BigDecimal Math - for high-precision decimal math
Groovy Floating Point Math - for high-speed decimal math
Lists and Sets - convenient notation to group items into a collection
Object Arrays - fixed-size arrays for faster collections
NEW Maps and Sorted Maps - convenient notation to assign values to keys
Groovy Data Structures:
Using Interceptors with the ProxyMetaClass - intercept calls to methods
Java Reflection in Groovy - how to examine and manipulate objects and their fields and methods when their names aren't known at compile time.