Natural law is a tradition that sees basic moral principles (called natural laws) as objective, based on nature instead of convention, and knowable to all through natural human reason. Natural moral laws differ from positive laws, which come from human legislation, and divine laws, which require divine revelation.Aquinas overview
These questions are about Chapter 13 of Harry Gensler's Ethics: A Contemporary Introduction (Routledge: 2011).
St Thomas Aquinas was the central figure in the natural-law tradition. His moral theology, which was based on his Christian faith, sees morality as part of God's governance of the world. Morality's purpose is to lead us to our final goal, which is eternal happiness with God. We have two ways to know the basic moral norms: human reason and the Bible. Morality is possible because of how God created us, as rational animals with an intellect and a will.Aquinas axioms
Aquinas's moral philosophy sees the basic moral norms (like the golden rule and the wrongness of stealing) as fixed and unchangeable, although they can be applied differently to different cultural situations. Human laws are set up to promote the good and must accord with natural law.
Good is that which all things seek after. The first precept of natural law is that good is to be done and pursued, and evil is to be avoided. We can know what is good by investigating our natural (rational) inclinations.
John Finnis, a follower of Aquinas, suggests that there are seven basic goods (which include, for example, knowledge and life), that these cannot be measured on a common scale and totaled, and that it's always wrong to choose directly against a basic good (for example, by taking the life of an innocent person).