Both of these are substitution variable adn difference 
between & and &&:

&--->the variable prefixed with an ampersand (&) to
prompt the user for a value.

eg:
SELECT employee_id, last_name, salary, department_id
FROM employees
WHERE employee_id = &employee_num ;

It should prompt the user to enter a value for employee_num


&&--->Mainly we used the  double-ampersand (&&) if you want 
to reuse the variable value without prompting the user each
time.

eg:SELECT employee_id, last_name, job_id, &&column_name
FROM employees
ORDER BY &column_name;

Here the user is asked to give the value for variable 
column_name only once. The
value supplied by the user (department_id) is used both for 
display and ordering of data.