The concept of natural law has its roots in the ancient pagan world. Which is not so surprising because natural law is, as St. Thomas says, "nothing else than the rational creature's participation in the eternal law."
Natural law is typically divided into three constituent parts, the discriminating norm, the binding norm (norma obligans), and the manifesting norm(norma denuntians).
The discriminating norm is the process by which natural law is discernible through the use of human reason, aided by the divine. The binding norm is the obligation to live in conformity to ones nature, in the universal order established by the Creator. The manifesting norm is reason which determines the moral quality of actions tried by the discriminating norm.
Much of our understanding of natural law was explored by St. Thomas Aquinas in The Summa Theologiae.
The notable factor of natural law as it applies to the modern Christian Catholic is that when applied to the matter of sin is that immoral actions which are discernible through the use of reason are by their nature not open to vincible ignorance.
Abortion is an excellent example. If we except, through the principle of natural law that killing of a human being is wrong, then the question becomes, as applies to the unborn, is a fetus a human being? This becomes a matter of ensoulment. A baby is a human being when it has a soul. When does this happen? The teaching of the Church is that a baby has a soul from the time of conception. Does this make sense through the discernment of reason? Well, we know that scientifically an individual human being is determined by their DNA. The DNA of each individual human being (with the possible exception of identical twins) is absolutely unique. So a baby, from the time it is conceived, is a unique organism. We are getting closer and closer to the time technologically when a fertilized egg (the first instance of the unique DNA combination which is a human being) will be able to, with technological support, be able to exist independently from its mother. So, by reason, that fertilized egg is in fact a unique independent organism, a human being. A human being with a soul. So through the precepts of natural law it is wrong to kill an unborn baby.
That being so, abortion is always wrong, and never open to invincible ignorance, and so always a mortal sin.