checked Exceptions must be dealt with in either a try/catch
block or by
declaring a "throws" in a method. Unchecked exceptions
normally are
Runtime exceptions like NullPointerException or
ClassCastException.

A simple rule of thumb: If it's an exception you can
possibly deal with
(continue to run the program using some alternative code),
use checked
exceptions. For exceptions that should never happen (if they
do, it's a
bug), use unchecked (Runtime) exceptions which will come up
to the
surface and displayed to the user. Like this you assure that
if there's
a bug, it will show up eventually and can be fixed, and you
don't run
the risk of catching an exception and forgetting to deal
with it (f.i.
empty catch block).