Really an ugly thing. A bill of attainder was a method used by legislative branches of yesteryear in order to control the population. Not like in birth control, but in manner and demeanor. Let's pretend you're saying some nasty things about the British Parliament in the 1600's. You're calling the House of Lords lots of nasty names and are generally being a nuisance by, say, attempting to start a revolt, or have a land dispute with a noble family. Parliament could then pass a Bill of Attainder and convict you guilty right then and there. No trial necessary, no nothing, just the votes of parliament.
While this doesn't sound like a bad thing, someone who starts a revolution is very obviously guilty of treason, but what about other things? Say you wanted to move your son up the political ladder, or a small Duke or Earl was not being helpful to you and your lands. With a little bribery, you could get parliament on your side and have them issure a bill of attainder against that individual. Bills were also issued posthumously, so that lands and properties of the dead could be seized. This wasn't done for everyone who died, just those who had opposing views to the current King or Parliament.
Bills of attainder were passed the same way that any other bill or law was passed. It had to pass both houses of Parliament. The accused, except when deceased, was allowed to present their case infront of Parliament.
Laws against Bills of Attainder started in the American Colonies, as the British Parliament started to use them against various people in an attempt to maintain control. Almost all of the Bills of Attainder held execution as punishment for the crimes of the individual. In Article I, Section 9, paragraph 3 of the Constitution of the United States of America it is written that there will be no bills of attainder, nor ex post facto laws in the USA. Yay! Go Constitution!
Bills of Attainder have been issued against: