Guide to Unix/Environment Variables
“Each program started from that shell will have VARNAME set to new value. The names of environment variables are case-sensitive; by convention they are uppercase. A shell variable is like an environment variable, except that it is not exported to new programs started from that shell. You could export it, but normally you just write a shell initialisation script to set it in each shell.”–via Guide to Unix/Environment Variables – Wikibooks, open books for an open world.