The LDS church believed in Jesus (and both Old and New testament) but has additional books of teachings as well and well as additional prophets. Wiki's brief synopsis of the differences is "Perhaps the most distinct difference between the LDS Church and other faiths is the belief that its founder Joseph Smith, Jr. was a prophet who received a visitation from God the Father and Jesus Christ ... and was directed to translate what became known as the Book of Mormon, a volume of scripture..."

So it's like Christianity plus a bunch of other stuff... a LOT of other stuff... and that other stuff is all things generally though of as heresy by other Christian sects. Actually, several sects consider the LDS church a cult rather than a church.