Like people, computers communicate in different ways. The simplest, sending notes or mail among users, is done with commands like mail. Mail is often restricted to small text files which means books and binary files should not be sent through the mail. There are Unix utilities such as encode, which converts binary files into encoded text files so they may be mailed-at least if they are not too big. If they are too big you must split the encoded file into several smaller files which can then be mailed.
How to best transfer files depends, naturally, on how the machines are connected. If both machines are connected to a local network or to Internet, the best choice may be ftp. The ftp program is widely available, powerful, and satisfying to use. It is useful for transferring files between machines with different operating systems such as between a Unix workstation and a VMS or MVS machine.
If the machines are at the same site and their system administrators cooperate, then more transparent communications such as NFS or rcp are available. In the other extreme are machines that are connected via modems and telephone lines or by serial, RS-232 lines. This is common for personal computers which use a modem to talk to a remote Unix workstation or mainframe. For these cases the choices for file transfer are limited to programs such as kermit and uucp. The kermit utility is available on many of the terminal emulator programs on PCs. The uucp utility is available on Unix systems and allows for the automatic transfer of files, including mail, between Unix machines via modems. This is one of the oldest methods of networking Unix computers and is still used in some locations.
Another choice for communications over telephone lines is available if both machines are using fast modems, 9600 baud or greater. This is the SLIP or PPP connection which can be configured to allow users to use the same networking commands on phone lines as they do on the ethernet.
An elementary and obvious fact to remember when establishing communications among any two computers is that they both must speak the same language, that is, the local and host computer must have the same transfer protocol. This transfer protocol permits automatic error checking and retransmission to correct detected errors as well as replacing the return and new line characters on one system by the appropriate characters on the other (too often an annoying problem). While you may be able to use commands like save stream on your emulator program to transfer files, this does not have error checking and, accordingly, should be considered unreliable for all but simple text. Further, if the two computers handle data at differing rates, you must use the slower rate on both machines for communication to occur. And finally, unless the two computers are running the same operating systems with compatible architectures, the files you transfer should probably be text or ASCII.