Environment variables are configuration options that can be set in a shell or other similar environments. All programs started from within the shell can read them and configure themselves.
Environment variables are key-value-pair configuration options like
FOO=bar. Environment variables allow you to configure a program before it gets executed and without the need to store these variables in proprietary configuration files.
They are present on all POSIX systems (Unix, Linux, BSD, Mac OS, DOS, Windows, ...). Environment variables can be set by any shell and read by nearly any programming language.
Some programming languages allow you to set Environment variables, but they only persist within the current process. You can't set them to be visible by the calling shell. So they are not useful for inter-process communication.