Propaganda is usually implemented with malicious intentions and lacks truth. Public relations involves using truthful information to put a positive spin on an issue, person, or organization.
Although the dictionary and textbook definitions of propaganda and public relations may be similar, the difference lies in the intentions and motivation in their usage. Both terms essentially refer to the spreading of information in order to influence others.
Propaganda, however, is typically used in a negative manner. It is often used with the intent to damage an opposing cause, organization, or individual. The ideas or information being spread for this purpose do not always have a foundation based in truth. Delivering false information or twisting facts in order to make them seem more sinister than they are would fall under the umbrella of propaganda. Political campaign ads designed to attack an opponent is an example of propaganda.
Public relations, on the other hand, is usually used to present truthful information in a positive light. Commercials and advertising would be considered public relations. When celebrities give interviews and appear on talk shows to promote a new movie, these activities would be considered public relations. When an individual or organization is facing a scandal or controversy, a public relations campaign may be put together in an effort to address the issue and restore the person's or company's reputation.