The most common use for an identifier is to name constants, variables, aliases, structures, subroutines and functions. You should try and use descriptive names for identifiers, as this will make your program easier to follow and debug. A valid identifier is composed of the following elements:

 [underscore] letter {letter | digit | underscore}

In other words: an optional underscore character followed by a single letter of the alphabet, followed by one or more letters, digits or underscores.

A reserved word has a special meaning for the compiler and cannot be used as an identifier. A list of reserved words can be found here.