How is monism different from monotheism?

Monotheism is a metaphysical duality by definition - in that it isolates a single god and everything else in the universe: as disparate entities.

Monoism (as indoctrinated by Sri AdiSankara's Advaita (and Spinoza's Monism too)) - believes in the oneness of all that there is in the universe.

Advaitic Monism is summed up by four of these aphorisms:

* ahambrahmasmi - "I am brahman" (where brahma == the eternal supreme consciousness of the universe

* ayamAtmabrahma - "The Atman is brahman" (Where the Atma is the self)

* prajnAM brahma - "Consciousness is the brahman"

* tattvamasi - "That Thou Art"

Shankara also quotes, famously in the vivEkacUdAmaNi - "Brahma satyaM jagat mithyA jIvO brahmaiva nAparaH" - literally, "Ultimately, there is no difference between the self and the eternal - They are one and the same".

This Advaitic consciousness however manifests to the unenlightened in deceptive facets of duality - which are labelled as 'prAtibhAshika' and 'vyAvahArika' respectively. The 'pArAmartha' level of realization is the truest and ultimate.

You can read more about Advaita here: http://en.wikipedia.org/wiki/Advaita_Vedanta

And about Spinoza here: http://en.wikipedia.org/wiki/Spinoza