Marketing is a term used to explain activities undertaken to create an organisation's markets. Its objective is to create and communicate with specific population groups likely to consume their product or service.
PR deals with publics. Publics organise around issues and seek out organisations that create those issues. These publics can range from pressure groups and government ministers to employees or the media.
Unlike marketing, money is rarely involved in opening discussion. PR does not seek to buy time or space to communicate a message but uses existing mediums to endorse that message by talking about it. So, for example, issuing a press release can generate editorial coverage and endorsement through the targeted media.
This enhances the relative value of PR above and beyond the cost of the media time or space secured. For the recipient, the fact that information has been driven by PR is often difficult to identify. As far as they are concerned, the information has been received through an authoritative and influential third party, giving it credibility and demanding attention.