In a network we could
Computers take turns sending packets, so packets ensure fairness. They also make it easier to do error detection: it is easier to check individual packets for errors and resend only bad packets, than to check a whole message for errors.
Considerations related to packets:
Each hardware technology defines the precise format of its packets. When we have a specific layout we speak of frames, and reserve the term packet for the general concept.
A typical frame has a header and a trailer so that the receiver knows where a frame starts and where it ends. Data stuffing is usually used so that the header and trailer bits do not appear in the data portion of the frame.
Error detection
How are packets (frames) transmitted? Depends on (1) physical characteristics (hardware) of the network and (2) the network topology.
LANs are characterized by nodes in geographical proximity that uses a single shared communications medium. This is cheaper than point-to-point (mesh) networks.
The term topology refers to the general shape of a network. Examples:
To coordinate shared access, bus-based LANs need to employ CSMA/CD or CSMA/CA or other such techniques; rings might pass tokens around.
WANs can span many continents. Unlines LANs, these tend not to have a single shared communications medium because they'd spend more time coordinating access to the medium than sending data. WANs are built from packet switches.
Each packet switch performs store and forward packet switching. Each packet switch has a routing table. Routing is source-independent and cares only about the "next hop." WANs can use static or dynamic routing. The routing tables are usually filled by running Dijkstra's algorithm to compute shortest (least-cost) paths.