An affirmative defense is one in which the defendant admits to the allegations against him, but claims that he was either justified or is not actually culpable for the crime. Killing in self-defense is a good example. If Fred jumps out of the shadows of a Miami Beach alleyway and attacks George with a knife, George might pull out his handgun and shoot Fred. If George is ever charged with Fred's murder, he will claim self-defense - although he killed him, it was justified.
Similarly, suppose Fred jumped out of the shadows of the home he shared with George on George's birthday, festively dressed in a party hat and shouting "surprise!" Most people would be appropriately startled, but correctly identify the beginning of a surprise birthday part. If George is severely mentally ill and paranoid, however, he may believe that Fred is there to harm him. In that case, if he shot Fred and was later charged with his murder, George would probably claim insanity as his best defense. Insanity is permitted as a legal defense because although the defendant committed the crime, he was not of sound mind at the time the crime occurred, and so cannot be found culpable for his actions.
If you have been charged with a crime anywhere in Florida, contact the criminal defense lawyers at Musca Law.