Identifiers refer to the names of variables, constants, functions and arrays. They are user defined names and consist of sequence of letters or digits. Both lowercase and uppercase letters can be used as identifiers. Identifiers are of variable length. Underscore character is also permitted in identifiers. Example : max,s234, no_of_days.
Every C word is classified as an identifier or a keyword. All keywords have fixed meanings and these meanings cannot be changed .Keywords serve as building blocks of a program statements .they can be used for user defined words. There are only 32 keywords in C whose strength lie in their rich built in functions.
Some of the keywords are:
auto double int struct break else long switch case enum register typedef char extern float return union const short unsigned continue for signed void default goto sizeof volatile do if static while