Murder, in its various degrees, is a charge applied when the suspect is thought to have killed a person with violent intent. There are differences according to the circumstance, but generally the assumption is that they meant for the person to die.
Manslaughter suggests an element of accident, that the suspect may or may not have meant to kill the person in question. The absence of malice or malice aforethought is an important consideration when the difference between the two charges is being considered.
The charge of manslaughter carries with it a lesser assumption of culpability in the case of the death of the person or persons involved and as such usually also carries a lighter sentence.