Positive law is made by people. Natural law comes from sources that are universal. To many people (for example, to Thomas Jefferson in the Declaration of Independence) the source of natural law is God. Natural law is universal; it applies to everyone. Positive law only applies to those people who are the subjects or citizens of the government that creates the law. Positive law must be written down. Natural laws are unwritten laws. In short, then, positive law must be made by a given government and it relies on the government for its power. Natural law is not made by people and has moral power regardless of whether a government recognizes it and makes it into positive law or not.
The distinction between natural law and conventional law is grounded in Greek philosophical thought, which distinguished nature (physis) from custom (nomos). Natural law, which was often associated with divine law, encompassed such things as the natural movement of the elements (earth descending and air rising) and the movements of the stars, and was part of nature; natural laws could not be changed by humans. Customs and conventional laws were created by humans, and included such things as social customs and criminal laws and could be changed by humans.
The Greeks, and many subsequent thinkers, made the point that if conventional laws were grounded in natural law then rather than being mutable and arbitrary, they would be fair and just. One sign of a law being "natural" is that it is universal rather than limited to one specific culture.
For example, an overwhelming number of cultures have taboos against incest. Modern evolutionary biologists argue that these taboos are rooted in the natural laws of genetics, because incest or inbreeding is likely to increase the frequency of deleterious recessive genes. On the other hand, most people think that laws and conventions determining gender roles, such as the prohibition against females driving in Saudi Arabia, are purely conventional, as not only are they not universal but not grounded in the actual statistical evidence that female drivers are actually far safer than male drivers, getting in fewer accidents and getting fewer tickets.
Natural law and positive law differ in a number of ways. Firstly, natural laws are God-given laws inherent in our being whereas positive laws are man-made. Secondly, natural laws are universal as opposed to positive laws which are only applicable to a geographically defined political territory such as that controlled by a government. Thirdly, whereas natural laws are eternal and constant, positive laws can be amended or rescinded. Fourthly, natural law is based on reason and human beings have the free will to choose what they feel is right or wrong. On the other hand, positive law prescribes what is right or wrong and people have to abide by the prescriptions. Positive law is enforced by institutions such as the police and judiciary. Lastly, natural law can exist even in the absence of man but positive law is dependent on the existence of man.