From WiKi, but with the links to the citations on the page:
Until reformed under the Treason Act 1814, the full punishment for the crime of treason was to be hanged, drawn and quartered in that the condemned prisoner would be:
1. Dragged on a hurdle (a wooden frame) to the place of execution. This is one possible meaning of drawn.
2. Hanged by the neck for a short time or until almost dead (hanged).
3. Disembowelled and emasculated and the genitalia and entrails burned before the condemned’s eyes.
4. The body divided into four parts, then beheaded (quartered).
Typically, the resulting five parts (i.e. the four quarters of the body and the head) were gibbeted (put on public display) in different parts of the city, town, or, in famous cases, in the country, to deter would-be traitors who had not seen the execution. After 1814, the convict would be hanged until dead and the mutilation would be performed post-mortem. Gibbeting was later abolished in England in 1843, while drawing and quartering was abolished in 1870.
http://en.wikipedia.org/wiki/Hanged,_drawn_and_quartered
From the Oxford English Dictionary -
— USAGE In modern English hang has two past tense and past participle forms: hanged and hung. Sometime after the 16th century hung replaced the earlier form hanged in general contexts, as in they hung out the washing, while hanged was, as it still is, retained for use in reference to execution by hanging, as in the prisoner was hanged.
— ORIGIN Old English.