Murder and manslaughter both mean that someone's life has been taken. Although the more serious crime is murder, which involves premeditation or a prior intent to take a life, a charge of manslaughter is a very severe charge. The difference between the crimes can be a very fine line and is often a cause of deep confusion with juries.
The intention to cause harm or death does not have to be present for a charge of manslaughter to be valid. In fact the Florida Supreme Court considers the act of "Culpable Negligence" to be manslaughter. It is defined as "the conscious doing of an act or following a course of conduct which any reasonable person would know would likely result in death or great bodily injury to some other person when done without the intent to injure any person but with utter disregard for the safety of others."
Another form of homicide, often termed vehicular manslaughter, is a variation of the two above. If an unlawful action is coupled with a driving related death, such as DUI or leaving the scene, then the car accident can be treated as a 1st degree felony. The penalties for this can be financially brutal and involve many years in prison and should not be taken lightly.