e1||e2||... is the logical OR function. It evaluates its arguments in order, giving True immediately if any of them are True, and False if they are all False.
Or[e1, e2, ...] can be input in StandardForm and InputForm as . The character can be entered as Esc || Esc, Esc or Esc or \[Or].
Or has attribute HoldAll, and explicitly controls the evaluation of its arguments. In the are evaluated in order, stopping if any of them is found to be True.
Or gives symbolic results when necessary, removing initial arguments that are False.