Char is a fixed length data type and varchar2 is a variable length data type.

 Lets test:

set serveroutput on

declare
  variable_ch char(32767) := ' ';
  variable_vch varchar2(32767) := ' ';
begin
  dbms_output.put_line('variable_ch is ['||LENGTH(variable_ch)||']');
  dbms_output.put_line('variable_vch is ['||LENGTH(variable_vch)||']');
  variable_vch :='Hello';
  dbms_output.put_line('variable_ch is ['||LENGTH(variable_ch)||']');

  variable_vch :='Hello';
  dbms_output.put_line('variable_vch is ['||LENGTH(variable_vch)||']');
end;
/

Output:

variable_ch is [32767]

variable_vch is [1]

variable_ch is [32767]

variable_vch is [5]