They have to have one or more fields in common for the join. Let's say they have two fields in common; field1 and field2. Then to join the two tables (t1 and t2) together on these fields use:
SELECT *
FROM
t1
INNER JOIN
t2
ON t2.field1 = t1.field AND t2.field2 = t1.field2
An INNER JOIN returns rows from t1 and t2 where both tables have matching fields (i.e. field1 and field2).
Lots of online help around. If this doesn't help, search on yahoo for "SQL JOIN".
Cheers,
BidNo