Mormonism is a religion, belief system, and culture originating from the Latter Day Saint movement in the early 1800s. Many who practice Mormonism call themselves Latter Day Saints or Mormons.
Mormonism is a form of Restorationism, and is a branch of Christianity (in the sense of acknowleding that Jesus Christ is the son of God and messiah and professing to follow Jesus' teachings), although many Christian denominations classify Mormonism as heretical Christianity or non-Christianity. See Mormonism and Christianity.
There are several religious organizations who practice religon derived from Mormonism most of these sects numbering as a few thousand, hundred or even less. By far the largest and most recognized organization is the Church of Jesus Christ of Latter-day Saints. Another notable organization is the Community of Christ. Among the small Mormon organizations, there are some usually isolated communities in Utah, Idaho, Arizona, and surrounding states who practice polygamy; however, these groups, and the current practice of polygamy, are rejected by most Mormons.