[help] subquery vs outer join

Table name: table_a, table_b
size of both tables: ~500M


case 1)
select a.feld1, b.field1 from table_a a, table_b b where a.field1 =
b.field1 (+)

case 2)
select a.field1,
(select field1 from table_b where field1 = a.field1) col
from
table_a a


Two cases will return same results but the response time of case 2 is much
faster than case 1 as I really don't understand why? Also what does Oracle
actually do on both case?
pls help, thanks.