"DIA DE MUERTOS" or "DAY OF THE DEAD" is one of the oldest and most important Mexican traditions. Day of the Dead is celebrated in Mexico the night of November 1st through November 2nd. It is a CELEBRATION wherein people prepare for the return of their deceased loved ones for that one night a year. Families and friends create offerings to the returning dead in the form of "altars" or "altares". 
The point of the altar is to create an offering of things that the returning deceased enjoyed in life so that they may enjoy them again for this special night. Their favorite food, drink, treats, items, music, flowers, etc... 
An altar always gives us a very clear idea of WHO the altar is dedicated to, by having photos of the deceased, personal articles that belonged to them or are associated with them. In recent years, many people have decided to also dedicate altars to more abstract concepts that still evoke loss or disappearance. There is no "International Commission on Day of the Dead Altars", so while it is important to understand the essence of Dia de los Muertos, when it comes to altars creativity is always given free rein. 
To learn more about Dia de Muertos altars click on the following links: