<b>here a is input and  b is output parameters.</b>

SQL> create or replace procedure p1
  2  (a in number,
  3   b out number)
  4  as
  5  begin
  6     select sal into b from emp where empno=a;
  7  end;
  8  /

Procedure created.

<b> out parameters are meant for front end apps.
but inorder to get these values in sql*plus u need to write another unit to read this. something like this.</b>

SQL> ed
Wrote file afiedt.buf

  1  create or replace procedure p2
  2  as
  3  out number;
  4  begin
  5  p1(7788,out);      
  6  dbms_output.put_line(out);
  7* end;
SQL> /

Procedure created.

SQL> set serveroutput on
SQL> exec p2
3000

PL/SQL procedure successfully completed.