The two PRINT procedures perform formatted output. PRINT performs output to the standard output stream (IDL file unit -1), while PRINTF requires a file unit to be explicitly specified.
If FORMAT is not specified, IDL uses its default rules for formatting the output. FORMAT allows the format of the output to be specified in precise detail, using a FORTRAN-style specification. See Using Explicitly Formatted Input/Output .
If the FORMAT keyword is not present and PRINT is called with more than one argument, and the first argument is a scalar string starting with the characters "$(", this initial argument is taken to be the format specification, just as if it had been specified via the FORMAT keyword. This feature is maintained for compatibility with version 1 of VMS IDL.