One way to quit vi is through :wq,
which will save the current buffer before quitting. You can also quit without
saving with :q or :q!. The
latter is used when you've modified the file but have not made any changes
to it.
On occassion, your machine might crash or vi might crash.
However, both elvis and vim will take
steps to minimize the damage to any open buffers. Both editors save the open
buffers to a temporary file on occasion. This file is usually named similarly
to the open file, but with a dot at the beginning. This makes the file hidden.
This temporary file gets removed once the editor quits under normal
conditions. This means that the temporary copy will still be around if
something crashes. When you go back to edit the file again, you will
be prompted for what action to take. In most cases, a large amount
of your unsaved work can be recovered. elvis will also
send you a mail (from Graceland, oddly enough :) telling you that a backup
copy exists.