What is a deontological ethical theory?

A deontological ethical theory is a theory which focuses on rules. Actions are right or wrong depending on whether or not they conform to the rule in question. For deontologists lying is wrong in virtue of the fact any good ethical theory would contain the rule 'Do not lie'. This rule applies regardless of what particular circumstances you find yourself in. Deontologists argue that it is wrong to lie even in cases where it may be better to lie (for example to help a friend). This is the defining feature of deontological ethical theories and helps us distinguish them from their main rivals: consequentialist theories (i.e. theories which assess the wrongness of an action in light of the consequences of acting in that way).