A pendulum is a small weight attached to a string, chain, cord or other string like material. It can be made from many things, including coins, rocks, knives and stones or crystals.
It does need to be of an equal weight all the way around, so that it has no offsetting on either side, causing it to pull by momentum.
The idea is to ask a question and based on the pendulum's movement, you will find the answer. Pendulums can be used alone or with cloths that have answers or images relating to answers on them. I have even used mine on a Ouija Board, Spirit Board and also on the palm of my hand, having each finger represent an answer. Just like with tarot, runes and bones, these are a great tool for answer seeking and direction.