Trees represent relationships between things, but those relationships are always hierarchical. Child nodes are "under" their parents and "at the same level as" their siblings. The concepts of moving "down" and "up" in the tree have a clear meaning.
In some situations, we want to represent relationships, but in a less strict way. For example:
In a transportation network, an intersection or a station may have multiple connections to other intersections. In this example, the links between items mean "is connected to" (physically) or "can travel from...to