According to UShistory.org, the Stamp Act of 1765 was an attempt on the part of Great Britain's Parliament to reduce the country's national debt, which had grown from £72,289,673 to £129,586,789 over the previous 10 years, largely as a result of the Seven Years War. The Stamp Act was the first serious attempt Parliament made to tax the colonies and assert authority over them.
As The Colonial Williamsburg Foundation points out, the Stamp Act levied a tax upon every printed piece of paper any American dealt with, including licenses, playing cards and newspapers. Because this was the first time Parliament attempted to raise money through taxing the colonies without any approval by any colonial legislatures, the colonies viewed the Stamp Act as a bad precedent. They surmised that if Parliament taxed them like this once, it could happen again.
According to About.com, the issue at stake for the colonists regarding the Stamp Act was that of taxation without representation. In response, 11 of the 13 colonies sent formal protests to Great Britain, and widespread boycotts of British goods began. The Stamp Act Congresses were convened in June 1765, and came to a resolution known as the "Declaration of Rights and Grievances," which stated that only colonial legislatures had the right to tax the colonies. As mob violence grew against the Stamp Act throughout the colonies, Parliament backed down, repealing the Act in 1766.