Unix system use a lot of environment variables. Most of them are initialized when you log in and are maintained until you log out. This is a list of the most usual/usefull variables to know. Keep in mind, depending on the OS/distribution and your shell, some may or may not exist:
PATH – List directories the shell searches for commands HOME – User's home directory TERM – Indicate which terminal emulator is used PS1 – Shell prompt in the Bourne shell and variants MAIL – Path to user's mailbox TEMP – Path to directory where processes can store temporary files TZ – Timezone settings PWD – Path of the current directory HISTFILE – Name of the file in which command history is saved HISTFILESIZE - Maximum number of lines contained in the history file HOSTNAME - System's host name LD_LIBRARY_PATH - List of directories where libraries should be searched for USER - Current logged in user's name DISPLAY - Network name of the X11 display to connect to, if available SHELL - Indicate current shell path TERMCAP – Display termcap database entry for the current term OSTYPE – Type of operating system MACHTYPE – The CPU architecture that the system is running on EDITOR – The user's preferred text editor PAGER – The user's preferred text pager MANPATH – List of directories to search for manual pages
You can also list all the environement variables on your current session, using the printenv command.