The Stamp Act of 1765 was abhorred by the colonists because it represented an effort by the British to use taxes in order to raise money, and not to regulate commerce as in the past. For the colonists, this set a troubling precedent that would open the doors for more extensive taxation in the future.
The Stamp Act was passed by the British government without the approval of the American colonies, and few colonists felt they had legislative recourse. The new legislature required them to pay a tax, or stamp duty, of between 2 pence and upwards of 6 pounds on every single piece of printed paper used. This included ships' papers, legal documents, newspapers and playing cards.
The act was essentially an attempt by the British to assert control over the American colonies. It was especially important as a means to recover losses after the Seven Years War, during which time the national debt rose dramatically, and English citizens, as well as those in the American colonies, were being heavily taxed.
It wasn't until the Virginia House of Burgesses partially adopted Patrick Henry's Stamp Act Resolves that the legislation was effectively challenged. The Resolves were a declaration of equal rights for Americans, stating, among other things, that they should only be taxed by their own appointed officials and representatives.