The two can achieve much the same effect.
(eg
SELECT
a.ID,
a.Fld
FROM a
INNER JOIN b
ON a.Fld = b.Fld
;
-- will produce the same as
SELECT
a.ID,
a.Fld
FROM a
WHERE a.Fld IN (
SELECT b.Fld FROM b
);
-- if table b contains three records in Fld: 'a', 'b', 'c', then also...