Modern Spiritualism as a religious movement emerged during the middle of the nineteenth century, reaching its peak around the turn of the nineteenth to the twentieth century. As a religion, it is not Christian, Jewish, Muslim, Buddhist, or Pagan, or even non-denominational. Spiritualism is its own denomination, defining a particular approach to spirituality.
Briefly, Spiritualism is a religion, science, and philosophy for life. Spiritualism states that life is continuous - in other words, we never die, we just change states of being. As a science, Spiritualism is committed to proving the continuity of life by communicating with spirits who have passed on. (For more information about the scientific underpinnings of these concepts, see veritas.arizona.edu.)
Spiritualists follow a set of principles but have no dogma. The principles serve as guidelines for living, such as "treat others as you would like to be treated" and "infinite intelligence (Spirit, God) is all there is." Rather than concepts of sin and redemption, or other specific dogma, Spiritualists believe that we have personal responsibility for our spiritual development and that our actions create the reality we live. Spiritualists strive to find truth and to live accordingly to those truths. (You can find a listing of the principles at the websites noted below.)
Many but not all mediums are Spiritualists. Not all Spiritualists are mediums. But there is a long relationship between mediumship and Spiritualism, and Spiritualist churches and camps have been the primary source of training for true mediums. As noted in the section on What is a Medium?, a medium is someone who serves as intermediary between people here on this earth plane and those in the spirit realm. A medium specifically "talks to dead people," as well as angels, pets, and spirit guides. A medium is one vehicle for proving the continuity of life. (See What is a Medium? for more information about the difference between psychics and mediums.)
For more information about Spiritualism, check out the following websites: