A tree is a node that may have children. Tree's are inherently recursive by definition as each child of a node is a Tree itself, with or without children nodes.
A tree is a special case of a graph structure as this is the only graph that have a hierarchical relationship.
It's an rooted acyclic undirected graph. Undirected because the relation between a parent and its child is symmetric.
Relationship rules:
A tree is a data structures with no or slow random access. Tree structure scale well.
A set of tree is known as a forest.