Packet is a digital system that allows computers to connect to each other via ham radios and exchange data. With packet, messages can be sent around the world. It is very easy for any technician with the proper equipment to send a message anywhere in the world. This form of e-mail is sent across oceans by HF stations. They can do this because people with multiple radios and packet modems can change the band a packet is sent on. For example, Kevin could send a message on two meters, and you could receive it on 440.
The space station MIR has a packet system. When it passes overhead, you can leave mail for the people aboard!
Packet uses a modem much like the one a telephone modem uses, however, most packet modems are in boxes on the outside of the computer. They connect a computer to a radio, which acts like the phone line sending and receiving data from a local packet station.
There are also packet bulletin boards. On these bulletins are posted for sale ads and personal e-mail. Packet Bulletin Board Systems (PBBSs) are where hams leave and send their mail. On the VHF and UHF bands, a PBBS is mostly what you use, there is not as much chatting as on the HF bands.