LAN's or local area networks are the lowest level. LAN's are interconnected computers typically associated with one organization. These computers connect to each other but not necessarily to the Internet. The computers connect to one or more other LANs comprising what is called a WAN or Wide Area Network. Note that this is usually not a simple one to one connection, thus it is often possible for data to travel numerous paths between two LANs.
Backbones are major cables or other communications links between various points.
They are high speed lines between places like the Bay Area and Chicago. Packets contain information on source, destination, content, and ordering which allow them to move through the Internet from gateway to gateway and find their way to a destination. It is possible for packets from the same document to take various paths and arrive at the destination in the wrong order. The protocol can deal with this. It can also deal with errors in the data and missing packets by requesting the sender to retransmit them.