The word indulgence (Latin indulgentia, from indulgeo, to be kind or tender) originally meant kindness or favor; in post-classic Latin it came to mean the remission of a tax or debt. In Roman law and in the Vulgate of the Old Testament (Isaiah 61:1) it was used to express release from captivity or punishment .
As this document is a work in progress, I will leave you with the following links that describe indulgences at great length. I thought it a better idea to link to these pages while I finish this document than leave the subject untouched.
Catholic Encyclopedia: Indulgences
Catholic Answers: Myths about Indulgences