The ARRL NTS (National Traffic System) is designed to operate from one local area to another local area (say, across the country) by the process of Relaying messages from one station to another. I am a member of several section nets in the northwest area.
If I had a message that I wanted to originate from the Seattle area to a destination across the country (for example, Miami, Florida) several different stations are involved that operate at different levels of the relay schedule. For example, I would post the message traffic to the CW section net called WSN (Washington State Net) with final destination to Miami (presume a phone number is the contact method).
In WSN the message would be taken from me by an operator who is scheduled to submit it to RN7 (area net). Then, an RN7 station will take the message and probably submit it to PAN (Pacific Area Net) to make its way across the country via dedicated and scheduled operators who move that traffic.
On the east coast, the reverse process may happen, the message will be taken by the EAN (Eastern Area Net) operator and submitted to 4RN for delivery to a section net closest to the Miami destination. The final operator who takes the message is usually local to Miami and makes the phone call to deliver the message.
For priority messages that must be delivered in a given time interval, this process can occur relatively fast assuming good propagation at each relay point. Sometimes QRN or other propagation problems can halt the process until the message can be sent. There are methods defined for handling messages that cannot be delivered on time which can include ACK or NAK messages relayed back the originating station.
In the US, virtually every ham radio operator has a local traffic net that they can join. Evening nets on the 80 meter band are usually the section nets or area nets that handle message traffic. Often, the PAN nets may pick up a message in the evening for cross country hop but wait until the next day for a scheduled hookup, usually on the 20 meter band.
Operating as an NTS volunteer traffic handling station is good experience, there are both CW and SSB HF traffic nets and even the VHF/UHF repeater stations host traffic nets.