I advise against the use of these as they can prevent your code from being portable. They refer to a single, specific thing such as the number 1.25 or the string 'Samuel Adams'. If you try to do anything foolish such as assigning a value to a constant, the compiler will complain and die. It is also possible to make a variable constant using the keyword const, or to create a symbolic constant using the preprocessor. We will discuss the concepts in an upcoming lesson.
Variables are more like pronouns.
You can use them in all the same ways that you could use a constant, but you can change which thing they are referring to. For instance, him might mean Dennis Richie in one instance, but Peter van der Linden in another. The first character must be a letter or an underbar. Variable names are guaranteed to treat the first 31 characters as significant, except in the case of external variables where only the first six characters are guaranteed to be significant. You may not use one of C's keywords as a variable name.