Mormonism is polytheistic; they believe that the Father, Son, and Holy Ghost are different gods, and that people are to become gods too.
Again, this looks to me like a matter of semantics, nothing more. Non-Mormon Christians speak of the Trinity comprising one god in three persons. Conversely, Mormons say:
Mormons believe there are three Gods, but who function as an entirely unified body. Together, they form the Godhead and fulfill the complete mission of God. The Supreme God is God the Father. He is generally the One the Christian Bible refers to when it speaks of God, although in some cases, including in the Book of Genesis, Jesus is referred to as God during the creation of the world. Mormons teach that Gods Son created the earth, under the direction of God the Father. However, it is God the Father that Mormons call God and worship as such. He is the one they consider their Heavenly Father, while Jesus is their brother, the Son of God. (mormonchurch.com)
Interestingly, while nowhere in Scripture does the word "trinity" appear, the "Godhead" that Mormons cite is mentioned at least three times, in Acts 17:29, Romans 1:20 and Colossians 2:9.
What's more, the Christian Bible repeatedly refers to multiple "gods," for example in 2 Chronicles 2:5:
"The house which I build is great: for great is our God above all gods...." This is hardly an isolated instance; see Exodus 15:11, Deuteronomy 10:17, Joshua 22:22, Psalms 82:1, Psalms 86:8, Psalms 95:3 and Psalms 135:5. At the very least, Christianity seems to be henotheistic -- acknowledging multiple gods, while worshiping only one.