How and when to use sys_refcursor in oracle
A cursor is a pointer to a result set for a query. By returning a sys_refcursor you allow the client to fetch as many or few of the rows from the query as it requires. In stateful applications this could be used to page through results. 

A cursor can allow more flexibility than writing a PL/SQL function that returns an array as it's completely up to the client how many rows to fetch and when to stop. That said, I've not found many cases where this additional flexibility is useful.

It's worth noting that the sys_refcursor is weakly typed, so you can return pointers to queries which not only have different from or where clauses, but different numbers and types of columns as well. Alternatively you can use strongly typed cursor where the columns in the result set are fixed. 

This enables you to write functions which return different queries, like so:

create function get_data ( type varchar2 ) return sys_refcursor as
  ret_cur sys_refcursor;
begin

  if type = 'EMP' then
    open ret_cur for select * from emp;
  elsif type = 'DEPT' then
    open ret_cur for select * from dept;
  end if;

  return ret_cur;
end;


However, if you're using sys_refcursor to create a generic "open a query" function like the above you're probably doing something wrong!