Joseph Smith, a convicted fraudster, started the Church of Jesus Christ of Latter Day Saints (LDS). People often call it Mormonism after the Book of Mormon, which contains several books in it, including the Book of Mormon. Since it’s often used in a derogative way, Mormons don’t like to be called mormon.
There are several offshoot church of the LDS, including the Fundamentalist LDS Church (polygamists, including the sister-wives TV show), the Community of Christ (RLDS), and many more.