Hiya! Well... Dowsing rods are generally used to find water, gold and oil below the ground. They're not used for divination too much as far as I know. Pendulums on the other hand are for divination. Basically, it's a weight on a chain/string of some sort. You hold it over your hand and you ask a question. If it's yes, the pendulum will swing one way. If it's no, the pendulum will swing in the opposite direction. So it's a way of getting yes/no answers. Here's a picture of what my pendulum looks like. It's rose quartz with chakra stones and the part you hold onto is a rose quartz heart:
What the site says about what it is (maybe this will help you understand better), "With a bob sculpted from a solid piece of rose quartz and a chain decorated with beads representing the 7 Chakras, this pendulum is a wonderful piece for your divination. Rose Quartz is a crystal particularly sensitive to emotional energies, particularly those involving love and as such, this pendulum is perhaps best suited for aiding in finding answers to emotional difficulties and questions concerning love. The conical bob measures approximately 1 3/4" in length and is affixed to a chain that measures approximately 8" long, easily fitting between the fingers to sway in guidance."
However, I would check out these websites for much more detailed information than what I was able to give you here:
Basic Pendulum Dowsing
http://dowsing-pendulums.com/
Pendulum Basics
http://www.reiki-for-holistic-health.com/pendulum.html
Pendulum Dowsing Instructions
http://diviningmind.com/dowsing-instructions.html
Pendulum Dowsing Techniques
http://diviningmind.com/energy-dowsing.html
Dowsing - The Master Resource Page
http://www.greatdreams.com/dowsing.htm
Dowsing With Your Pendulum
http://www.greatdreams.com/penlearn.htm
Free Pendulum Dowsing Charts
http://pendulumcharts.com/charts.html
There is also a REALLY great book out... It's called, "The PENDULUM Bridge to Infinite Knowing: Beginning Through Advanced Instruction Complete With Pendulum Charts" by Dale W. Olson
Hope all that helps! Also, if you have any specialized questions, just post a thread - there's plenty of people on here willing to discuss this topic and answer questions. You can also send me a private message if you like :)
Bright blessings!