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 then used as a charm to protect sleeping
children from nightmares. Dreamcatchers made of willow and sinew are not meant
to last forever but instead are intended to dry out and collapse over time as
the child enters the age of adulthood.

The Ojibwa believe that a dreamcatcher filters a person's dreams. According
to Terri J. Andrews in the article "Legend of the Dream Catcher," about the
Ojibwa nation in the magazine World & I, Nov. 1998 page 204, "Only good
dreams would be allowed to filter through . . . Bad dreams would stay in the
net, disappearing with the light of day."

-Wikipedia