Data Tree
Complex data in Grasshopper, that is, data that is more structured than a singleton or a simple list, is stored in a "data tree".
Part of Grasshopper
Part Type Entity
Screenshot Grasshopper Data Tree.png

Resources

Data Tree Editing
TUDelft web resource presenting data trees and how to manipulate them.
Working with Data Trees
A ModeLab introductory tutorial on Data Trees - skip to the last video for the heart of the discussion.
Advanced Topics in Grasshopper
A discussion by David Rutten on advanced tools for working with Data Trees. Topics include: Editing data tree structures, Custom mapping functions, Offsets within and between trees, Selection rules for items in data trees.