"The hands of friendship, crown of loyalty and the heart symbolising love. The tradition dates back to the 16th century when a native of Galway City, Richard Joyce was captured by Algerian Corsairs on his way to the West Indies. When he eventually returned he settled in the small village of Claddagh just outside the city of Galway where he crafted the first Claddagh ring."
The interesting part is how the ring is worn...
Worn on left ring finger with the crown pointing towards your fingertips indicates you are spoken for, or married.

Worn on the right ring finger with the crown pointing towards your wrist, however, means that you are single, or that your heart is not spoken for.
You can find these rings in many gift shops and at almost any jeweler. However, I was told that I should get the ring from Galway where it originated (more authentic). There are a number of different styles one can choose from-some are very simple and some can get quite extravagant with engravings and different coloured gems inside the heart and around the band. Most all are sterling silver.