This includes all living things, and rocks and things are considered to be among living things. It also includes oneself. This is the biggest part of the concept that was designed to stop the persecution of practicing witches. In many of the older witchcraft traditions, it is not necessarily considered inappropriate to do harm to another, providing that one has just cause for inflicting the harm. Such as protecting one's life or family or possession etc.
Different traditions have their varying viewpoints on where the line is crossed.
The law of return is the idea that whatever we do, either for good or ill, is returned to us three times over. This is an attempt to keep the wiccans on a course for the good. Certainly it must be pretty important to expend negative energy on something, if you will reap three times that negative energy back into your own life. There are many who practice varying degrees of these ancient traditions. Included are all of the Native American traditions.