Q1. No difference. You can check it using profiling and compare execution plan.
Q2. As I know, only oracle support it. But it is not recommended to use in latest version of Oracle RDBMS:
Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions, which do not apply to the FROM clause OUTER JOIN syntax:
Q3. Oracle "invent" (+) before outer join was specified in SQL ANSI.