As @hvd says, the "where" clause filters rows returned by the join, so the "where" version won't return outer-joined rows (which have a.id = null).
However there is another significant difference: Even if the outer joined rows were not filtered out, there can be a massive performance boost putting the condition into the "on" clause, because the result set is made smaller earlier.
This is particularly pronounced when a series of other left joined tables follows the one with the "and" condition - you can prevent joins from even happening to the following tables for unsuitable rows and potentially chop off millions of rows from reaching the filtering ("where") stage.
