This page displays information abot the environment space.

  The environment space is used to hold information for programs to use. It
is controlled by using the DOS command SET. Some variables are commonly
set:
COMSPEC: This is the program that controls the DOS prompt. It is also used
         when you 'Shell to DOS' from programs.
PATH:    These are the directories DOS will search when you type a command
         it doesn't internally know. It first searches the current
         directory, then all of the ones listed on the PATH. It tries each
         directory by looking for a file with a .COM, .EXE, or .BAT
         extention, in that order.
PROMPT:  This is used to display the prompt.

  Many programs use the environment to hold information, such as where
their startup files are, and startup commands.