Each word of C is classified as a word or identifier. All keywords have specific meanings and these meanings cannot be changed. The keyword serves as the basic building block for program statements. A list of all keywords in C is listed in the table below. All keywords must be written in lowercase. Some compilers may use additional keywords that must be identified from the C keyword list.
Identifiers specify the names of variables, functions, and arrays. These consist of a sequence of letters and numbers, including a user-defined name and a letter as the first letter. Both uppercase and lowercase letters are allowed, although lowercase letters are commonly used. The underscore character is also allowed in the identifiers.