Pantheism, a type of monism, is the view that the Universe (Nature) and God are identical - that everything is Nature. Pantheists thus do not believe in a personal, anthropomorphic or creator-god. The word derives from the Ancient Greek: "pan" meaning ‘all’ and "theos" meaning ‘God’. As such, Pantheism denotes the idea that god is best seen as a way of relating to the universe. Although there are divergences within Pantheism, the central ideas found in almost all versions involve the Cosmos as an all-encompassing unity and the sacredness of Nature.
As Richard Dawkins wrote, "Pantheists don't believe in a supernatural god at all, but use the word, god, as a non-supernatural synonym for Nature or for the universe or for the lawfulness that governs its workings. Deists differ from pantheists in that the deist god is some kind of cosmic intelligence, rather than the pantheists' metaphoric or poetic synonym for the laws of the universe. Pantheism is sexed-up atheism, and Deism is watered down theism."
If Pantheism means nature, it seems as if it would be simpler to call nature "nature" and not confuse the issue by calling it god. It adds no new information to use god in this sense. On the contrary, it muddies the water. Besides, a god who is nothing but the laws of nature and is synonymous with "reality" is not the kind of god that most people develop any sort of passion for - you can't pray to it, it won't watch you as you live your life, it doesn't care about you, it can't intervene on your behalf, and it doesn't do miracles.
Panentheism, on the other hand, holds that God contains, but is not identical to, the Universe. God contains attributes that are not visible to humans in the universe, but that a portion of his presence pervades the universe. See the Spinoza reference next in this document.