It is copied from a Japanese movie so the motivations are different than those of Western films.... yes, both the woman and Sadako are a type of Yūrei (ghost) known as Onryō
http://en.wikipedia.org/wiki/Onry%C5%8D Some of these continue to haunt long after their killers are dead, simply by the inertia of their sense of vengence. Sometimes particularly strong emotion can create Ikiryō (not a ghost but behaves like one) while the person is still alive (it becomes a separate entity from the person who creates it, born of intense rage, remorse, frustration etc.) They sometimes seem like an evil twin of the person who spawned them.
Ju-on/Grudge has sort of a kharmic twist in that the man killed his wife in such a brutal way that she came back as an onryō allowing his evil influence to live on after his own death... the little kid is a zashiki-warashi or child ghost. These often cause a lot of minor mischief, but are generally believed to bring good fortune and protect the home, but in this case he's probably hanging around because the Onryō is his mother and he doesn't have the power to counteract her or her father's evil kharma. Anyway, that's my take on it.