Child abuse is the targeting of children for violence. It can often take the shape of using children for sexual purposes.
Kinds of Child Abuse
There is a qualitative difference between the kinds of abuse children can suffer at the hands of their parents, the kinds of abuse they can suffer at the hands of extended family members or family friends, and the kind of abuse they can suffer from strangers.
Typically, parental child abuse involves neglect and non-sexual violence. Parents have a responsibility to their children to provide adequate food and shelter, in addition to providing for their social and emotional needs. When a parent deprives their child of any one of these things, this is neglect, a form of child abuse. While physical discipline is an option some parents reject, it is not a form of abuse, and indeed can be beneficial to the child in the long run, according to some psychologists. But when a parents violence towards their children progresses beyond spankings or other traditional means of physical discipline, and becomes punching, kicking, or physical punishment for minor or imagined transgressions, this constitutes child abuse. Parents are rarely accused of sexual abuse against their own children, although this is not unheard of.
When a child is abused by a family member who is not a parent, or a family friend, it is very often sexual in nature. Child sexual abuse is one of the worst crimes a person can commit. While parents often imagine pedophiles to be strangers intent on kidnapping children from playgrounds, more often children are abused by family members or friends trusted to babysit or otherwise be left alone with the child.
Abuse by strangers is not common - that vast majority of abuse is done by parents, and the vast majority of cases which do not involve parents are perpetrated by other family members or family friends. When a stranger does abuse a child, the abuse tends to be more severe and intense than can be received from family members or friends, and more often ends with the death of the child.
Causes of Child Abuse
The use of illegal drugs and alcoholism is strongly correlated with an adult abusing a child. Two-thirds of all reported cases of child abuse involve an abuser who used drugs or alcohol. However, less obvious factors are suspected to influence an adults propensity towards child abuse. Adults who were abused as children are much more likely to abuse children themselves. In relationships which involve domestic violence, both the parents (ie, both the abuser and the abused) are more likely to abuse the children than parents in non-violent relationships are. There are also higher rates of child abuse against children with physical or mental handicaps.
Child abuse is least common in middle-class (or richer) homes with happily married parents. Families which are involved (as a group) in the local community, often through a local parish community, are less likely to have abusive parent-child relationships. 
Effects of Child Abuse
...child abuse is coercive: the abuser preys upon a child’s moral and intellectual helplessness. The abuser gets all the self-satisfaction he or she wants and in the process leaves the child with a life-long emotional scar of having been exposed to the manipulative aspects of life well before having developed healthy defense mechanisms to cope with such psychological assaults. The abuser walks away smacking his lips, and the child is left as bones for the garbage.
—Raymond Lloyd Richmond, Ph.D., 
The effects of physical violence on children are significant.
Children who are targets of sexual abuse suffer profound developmental changes as they grow into adults. Women who were abused as children are 3.8 times more likely to smoke cigarettes and indulge in other dangerous and harmful habits than women who were not abused. Men who are abused as children are ten times more likely to attempt suicide. Unfortunately, two out of every ten girls, and one out of every ten boys, is sexually abused before they are 13 years old.
Child Abuse Law
In the United States, every medical and mental health professional who discovers evidence of child abuse in the course of performing their duties is required by law to report the suspected abuse to authorities. These laws also apply to social workers and teachers. In some states, these laws extend to clergy and attorneys, and in only 18 states, the mandatory reporting laws extends to anybody who discovers evidence of child abuse.
- Famularo et al, Parental substance abuse and the nature of child maltreatment, Child Abuse & Neglect, vol. 16 iss. 4 pg. 475–83