View image A tree is a non-linear data structure where the data items can be thought of as occurring at different levels. There are links between items at one level and their descendants at the next. The data items are usually called nodes with the links known as branches. A node may have any number of descendants but itself may only be the descendant of one other node. A parent node is the node immediately above a given node, at the next level up. There can only be one parent node for each node, but different nodes, known as child nodes, may share the same parent.