Homicide is when one person causes the death of another person. This does not necessarily mean murder. There are many different forms of homicide. For example, vehicular homicide includes a person accidentally killing another person in a car accident. In order for a homicide to be considered murder, there must be intent to kill.
Some forms of homicide are considered criminal acts, and others are not. For example, if a person kills another person in self defense when there was a serious threat on the person's life, then this is generally not considered a criminal act. Soldiers and police officers also are legally authorized to commit homicides in certain circumstances.
Manslaughter is a form of homicide where there was no intent to kill, but the person's actions were still considered illegal and that can lead to a criminal charge. For example, if a person committed a homicide while driving under the influence, it could lead to a more severe criminal charge.