See the following documentation for more discussion.
Variables can represent lists of file names, options to pass to compilers, programs to run, directories to look in for source files, directories to write output in, or anything else you can imagine.
A variable name may be any sequence of characters not containing :, #, =, or leading or trailing whitespace. However, variable names containing characters other than letters, numbers, and underscores should be avoided because they may be given special meanings in the future, and with some shells they cannot be passed through the environment to a sub-make (see Communicating variables to a sub-make ).
Variable names are case-sensitive. The names foo, FOO, and Foo all refer to different variables.
It is traditional to use uppercase letters in variable names, but we recommend using lowercase letters for variable names that serve internal purposes in the makefile, and reserving uppercase for parameters that control implicit rules or for parameters that the user should override with command options (see Overriding variables).
A few variables have names that are a single punctuation character or just a few characters. These are the automatic variables, and they have particular specialized uses. See Automatic variables.