The Unix mail has been around for years, even before networking existed. mail is a simple interface to send messages and small files to other users, who can then save the message, redirect it, reply to it and such.

The mail command is still useful, e.g. to mail the output of a command to somebody:

cat cv.txt | mail

The elm mailreader is a much needed improvement to mail, and so is pine (Pine Is Not ELM). The mutt mailreader is even more recent and offers features like threading.

For those users who prefer a graphical interface to their mail (and a tennis elbow or a mouse arm), there are hundreds of options. The most popular is probably Netscape Mail. Gnome offers an MS Exchange clone. There are also tens of webmail applications available.