Michael Jackson died of an overdose of an anesthetic called propofol with other medications. Dr. Murray had given Jackson propofol and other drugs to help him sleep. Dr Murray was supposed to be monitoring Jackson while he slept. Propofol is not usually used as a sleep aid, but as an anesthetic during surgery.
In California, a person commits involuntary manslaughter when he does something that is a misdemeanor or is a lawful act done in an unlawful manner. The person must have committed the act with criminal negligence. Finally, the person's act must have caused a death.
"Criminal negligence" is a key component of involuntary manslaughter. A person acts with criminal negligence, when he acts with something more than ordinary carelessness, inattention, or mistake in judgment. Criminal negligence is when a person acts recklessly, creating a high risk of death or great bodily injury. The jury must also find that a reasonable person would have known that the act done would create such a risk.
Criminal negligence is also described as acting so different from how an ordinarily careful person would act in the same situation that it amounts to disregard for human life or indifference to the consequences of his act.
Sometimes it is unclear if the person's act actually caused the death. An act causes death if the death is the "direct, natural, and probable consequence" of the act, and the death would not have happened without the act. A natural and probable consequence is one that a reasonable person would know is likely to happen if nothing unusual interferes.
The prosecution here thus has to prove that Dr. Murray committed an act that was criminally negligent and caused Michael Jackson's death.