This is a wonderful online exhibit by the National Hispanic Cultural Center:
http://diadelosmuertos.nationalhispaniccenter.org/
Excerpt from the front page:
Cultures develop unique means of coming to terms with death. Anthropologists have traced ancestor worship and death rituals back to ancient Sumaria, Egypt, China, and Celtic Europe. In Mexico Día de los Muertos (Day of the Dead) celebrations have developed to such an extent that they are widely considered an integral part of Mexican identity. Día de los Muertos traditions carry a uniquely Mexican stamp, but in essence they are a fusion of pre-Columbian rituals and European beliefs brought by the Spanish to Mesoamerica.
Día de los Muertos is a folk tradition reflecting the folkways and folklore of Mexico and the identity of its people. As such, it is continually evolving and integrating newer elements as it crosses borders or as newcomers cross over to adapt these Mexican rituals into their lives. At the core Día de los Muertos traditions and rituals retain the primary mission of honoring, remembering and celebrating the life of all those who have come before us; as well as giving hope to our own inevitable mortality.