What is the difference between murder and voluntary manslaughter?

"Murder is the unlawful killing of a human being . . . with malice aforethought." (Pen. Code, 187, subd. (a).) In contrast to murder, "[m]anslaughter is the unlawful killing of a human being without malice." (Pen. Code, 192.) Voluntary manslaughter includes the unlawful killing of a human being on a sudden quarrel or heat of passion. (Pen. Code, 192, subd. (a).) In People v. Rios (2000) 23 Cal.4th 450 (Rios), our high court provided an in-depth analysis of the difference between murder and voluntary manslaughter. " 'Generally, the intent to unlawfully kill constitutes malice. [Citations.] "But a defendant who intentionally and unlawfully kills [nonetheless] lacks malice . . . when [he] acts in a 'sudden quarrel or heat of passion' ([Pen. Code,] 192, subd. (a))" '' . . . . [] These mitigating circumstances reduce an intentional, unlawful killing from murder to voluntary manslaughter 'by negating the element of malice that otherwise inheres in such a homicide [citation].' [Citation, italics in original.] Provocation has this effect because of the words of [Penal Code] section 192 itself, which specify that an unlawful killing that lacks malice because committed 'upon a sudden quarrel or heat of passion' is voluntary manslaughter." (Id. at p. 460.)