What are the Bind Variables & Substitution Variables?

Bind Variables are also called host variables, which is accessed even after the PL/SQL block is executed.
It can be create using VARIABLE keyword and referenced with a preceding colon-:.

ex.
VARIABLE sal number;
begin
select salary into :sal from emp where emp_id = 11;
end;
/
select firstname , lastname from emp where salary = :sal;

Substitution Variables are used to get user input at run time to avoid hard-coding of values that we can provide run time also.
We can reference the same with in PL/SQL block with symbol ampersand-&.

VARIABLE sal number;
accept empid prompt 'provide emp id' 
declare
v_empid number := &empid; 
begin
select salary into :sal from emp where emp_id = v_empid;
end;
/
select firstname , lastname from emp where salary = :sal;