Traditionally, the Ojibwa construct dreamcatchers by tying sinew strands in a web around a small round or tear-shaped frame of willow (in a way roughly similar to their method for making snowshoe webbing). The resulting "dream-catcher", hung above the bed, is used as a charm to protect sleeping children from nightmares. As dreamcatchers are made of willow and sinew, they are not meant to last forever but are intended to dry out and collapse as the child enters the age of adulthood.

Source: Wikipedia