How is the LDS churches belief on a pre-mortal existence different than other Christian religions? A document published by the church Of Jesus Christ of Latter Day Saints titled the proclamation to the family  refers to a premortal identity and purpose or a premortal realm.

How is the LDS churches belief on a pre-mortal existence different than other Christian religions?
Most Christians believe that your spirit is created the same time as your body when you are conceived (or sometimes born depending on who you ask). LDS believe that before the Earth was even created, everyone's spirit was already created and we all lived together with heavenly father as spirit children. When we are conceived on Earth, our spirits enter the body. LDS also believe that we are literally spirit children of God the Father and are literally spiritually related to each other. LDS also believe that Jesus Christ is our oldest spirit brother (the first born of God) and that even Lucifer (Satan) was also our brother. There are a lot more. If you really want to know more, tackle one of those missionaries as they ride past on their 10 speeds and ask more. It really is an interesting concept even if you don't believe in other Mormon doctrine.