To be guilty of murder you must intend to kill, or to cause grievous ("really serious") bodily harm. It must be proved that the death was not due to your use of reasonable force in self-defence or for preventing a crime.
Everyone convicted of murder is sentenced to life imprisonment, with either a set minimum number of years before you will be considered for release, or a direction that "life means life". If you are tried for murder and acquitted, you may be convicted of manslaughter instead if the facts support this.
Manslaughter covers a wide range of circumstances, ranging from near-murder to near-accidental death. The range of penalties for manslaughter is equally wide, but prison is almost inevitable.
What would otherwise be murder may be manslaughter if the killing was due to provocation or was part of a suicide pact, or the accused person was suffering from "diminished responsibility" due to a mental disorder.
Causing someone's death without meaning to kill or because of a serious injury can be manslaughter. This may be if the death was due to really serious carelessness by someone who was under a duty to take care ("gross negligence"), or an unlawful act which all reasonable people would realise would involve a risk of harm.
Corporate manslaughter is, broadly speaking, killing caused by gross negligence on the part of an organisation, such as a railway company or a police force. It is punishable only by a fine.
Causing or Allowing the Death of a Child or Vulnerable Adult
Where a child or vulnerable adult has died and at least one member of the household might have caused the death, any member of the household can be convicted of allowing the death even if it cannot be proved which of them caused it.
To be guilty of this offence, it is necessary to have failed to take reasonable steps to protect the child or vulnerable adult from a significant risk of serious physical harm. The maximum penalty is 14 years in prison.