Branches and connections
A folder system is a tree: one folder can contain smaller folders, which can contain more. Each item has a place in a branching structure. An organisation chart is another example.
A graph shows connections more freely. A map of train stations and routes is a graph: stations are points and routes link them. A station can connect to several others, and routes may form loops. Software uses graphs for things such as route finding and social connections. Algorithms can search these structures for an item or a path.