HOME this is the pathname of your "home"
directory when you login and to which cd changes when invoked with no argument.
PATH defines the directories searched when
the shell is looking for the file to execute when processing a command.
MAIL if set to the name of a mail file the
shell will inform you of the arrival of new mail.
PS1 contains the shell prompt string.
PS2 is the continuation prompt when a command
spans more than one line.
IFS (Internal Field Separator) The set of
characters used for blank interpretation in the shell. Normally space, TAB and newline.