Ethics is the branch of philosophy that explores and lays out the proper way to live. Most ethics are concerned with how people should treat each other. The most generally agreed upon statement of ethics is some variation of "Treat others like you'd like to be treated." Beyond this, however, ethical systems can differ sharply, depending on other axioms. For instance, from a secularist perspective it's necessary that the government doesn't privilege the expression of a particular variety of religious belief, because that unjustly excludes a portion of the population from full participation in public life on grounds that are arbitrary. However from certain fundamentalist perspectives, such forcible imposition of ideas may be permissible, or even necessary, to avoid the government giving support to immorality and thus putting citizens in risk of going to Hell. Ethics is often divided into normative ethics, meta-ethics, and descriptive ethics.
 Normative ethics
Normative ethics is the field of ethics that explores what humans morally ought to do. Most normative ethical theories belong to either the Deontological or Consequentialist families. Deontological theories emphasize duty to a moral code, while Consequentialist theories emphasize good consequences. Deontological philosophies include Divine command theory (very popular among the religious) and Social contractarian ethics (popular among secularists). Consequentialist theories include Utilitarianism (the philosophy of choice for many Humanists) and Ethical egoism (the source of Ayn Rand's Objectivism). Generally speaking, all fundamentalist and evangelical Christians, and most other conservatives as well, adhere to Deontological ethics, while most atheists and religious liberals adhere to Consequentialism. The one notable exception is the social contract.
Meta-ethics is the study of the meaning of ethical statements. At the highest level, meta-ethical theories are divided into cognitivist (which state that moral claims can be true or false) and non-cognitivist (which state that they cannot). Cognitivist theories are further divided into realist (there are objective moral facts) and subjectivist (morality is relative). Forms of realism include ethical naturalism, which sees morality as reducible to non-moral facts, and non-naturalism, which sees morality as coexisting with the physical world, as in intuitionism. Subjectivists usually adhere to either Divine command theory or Objectivist ethics.
 Moral Relativism vs Moral Absolutism
Ethicists are also divided by how strictly they obey moral "laws". Those who believe that Christian (deontological) morals must be applied universally despite the fact that these morals are not universally accepted are called moral absolutists. Those who believe that morals ought to be adapted to situations to produce the best outcome (usually consequentialist) are called moral relativists, not in any way related to the Theory of Relativity.
 Descriptive ethics
Descriptive ethics is the study of moral development in humans, such as Auguste Comte's study of the morality over history. Descriptive ethics does not seek to pronounce on what is right or wrong, merely to describe what people think is right or wrong, and to investigate how ethical beliefs vary across time and space.
 Applied Ethics
Applied ethics is the branch of ethics which seeks to apply basic ethical principles (from normative ethics) to concrete situations, especially to controversial social issues such as abortion, animal rights, euthanasia, homosexuality, poverty, the environment, etc. However, given the lack of agreement on normative ethics, it is unsurprising that different normative ethical theories produce different results when we seek to apply them to concrete situations.
 Ethics without Religion
Fundamentalist Christians often claim that ethics are bunk without The One And Only True God as a starting point from which right and wrong can be derived. However, such Christians clearly know very little about ethics, as there are several ethical theories which derive "good" from other things. Such theories, known as Secular Ethics and praised by people as diverse as Richard Dawkins and the Dalai Lama, are a major thorn in the side for self-important religionists who want to feel like they are the only good people. The Social contract, Utilitarianism/Prioritarianism, Immanuel Kant's categorical imperative, Ethical egoism and altruism, and moral intuition are all sufficient substitutes for divine command. Many secular ethical theories go along very well with Secular Humanism, a much broader philosophy.