Deontology refers to a group of moral views that focus on rules or prohibitions for action. Deontologists hold that these rules have moral importance that is independent of their effect on the good (consequentialism) or our character (virtue ethics). Different deontological theories focus on different concepts, but many focus on the intrinsic moral value of principles like justice, rights, and duties.
In contrast with deontologists, consequentialists hold that principles like justice, rights, and duties are only instrumentally valuable, that is to say these are only morally important considerations if acting in accordance with them will lead to the best outcome. Thus the difference between deontology and consequentialism is most strongly felt in cases where we can violate a deontological rule, like “do not murder”, in order to bring about great benefits to other people. For example, would it be morally permissible for a doctor to painlessly, secretly kill a patient, in order to use their organs to save five other patients, who would otherwise die without the procedure? Most deontologists would argue that it is impermissible to kill the one to save the five, even if this would lead to a better outcome, because the doctor would be violating the moral prohibition against killing innocent people.
Note that consequentialists may follow rules or guidelines instrumentally, in order to promote the good (see naive consequentialism vs. sophisticated consequentialism).
Alexander, Larry & Michael Moore. 2016. Deontological ethics. In Edward Zalta (ed.), Stanford Encyclopedia of Philosophy.
Wikipedia. 2016. Deontological ethics.