Difference between ON and WHERE in JOINs

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