Examples of environment variable names

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.