As we're writing a program, we'll have to choose names for variables, functions, macros, and other entities. These names are called identifiers. In C, an identifier may contain letters, digits, and underscores, but must begin with a letter or underscore. (In C99, identifiers may contain certain “universal character names” as well.)
C is case-sensitive: it distinguishes between upper-case and lower-case letters in identifiers. For example, the following identifiers are all different:
job joB job jOB Job JoB JOb JOB.