Difference between ON and WHERE in JOINs - Mailing list pgsql-general

I'm looking for an article that explains the difference between these
constructs IN POSTGRESQL (the rules seem to differ from one DB to another) :

SELECT A.*
FROM A
JOIN B ON a.id=b.id AND A.somefield='somevalue'

and

SELECT A.*
FROM A
JOIN B ON a.id=b.id
WHERE A.somefield='somevalue'


I have noticed big differences though I don't know the rules and I've
been bitten several times recently. Time to learn.

Thanks,

JC