Identifier
"Identifiers" or "symbols" are the names you supply for variables, types, functions, and labels in your program. Identifier names must differ in spelling and case from any keywords. You cannot use keywords as identifiers; they are reserved for special use.
Constants
A "constant" is a number, character, or character string that can be used as a value in a program. Use constants to represent floating-point, integer, enumeration, or character values that cannot be modified.
Variables
"Variables" are simply storage locations for data. You can place data into them and retrieve their content.
For example:
int count;
char key;
char lastname[30];