Put simply, there are ceremonial laws (for instance, dealing with sacrifices) and moral laws (don't kill).
The Ceremonial Laws:
All the ceremonial laws (regarding cleanness and sacrifice etc) have been fulfilled in Jesus. The book of Hebrews explains this.
The Moral Laws:
The principles behind the moral laws are still binding (not as a means of salvation, but simply for right living). However, particulars about the laws are not universally binding, but may be conditioned upon circumstance or context.
For example, one moral law in the OT was that you had to have a fence around the roof of your house. Why? Because people often had events on the roof of their house. So, for instance, I might have a social gathering at my house and someone might fall off and injure themselves or worse. Of course we don't usually have parties on the roof of our houses anymore, so that particular law has become irrelevant. Nevertheless, the principle behind the law (that we should be concerned for the safety of others when they are on our property) still applies.
Also, some might argue that the penal laws no longer apply, because Israel was a unique theocratic kingdom. As such, it had the right to punish certain sins in a way that no other earthly kingdom does. So the particular punishment for rape was death. However, some would say no longer have the jurisdiction to issue the death penalty for that crime (or others) even though rape remains a moral violation.
P.S. The moral/ceremonial distinction is not a clean one, since ceremonial laws were morally binding on OT Israelites. However, it is the case that a subset of the moral laws served a ceremonial function and that Christ has fulfilled that ceremonial function so that there is now nothing for us to do in regards to them. So while some may object to the moral/ceremonial distinction on the grounds that the ceremonial was moral, this "objection" is really irrelevant since some moral laws still had a peculiar ceremonial use.
