Commercials are concise snippets of various products and services. Companies buy airtime so that they can sell their products on the air. Television, movies, and magazines demonstrate these "snippets." Commercials are put in specific demographics and forms of entertainment to get the message out about the product. It increases the chances of higher sales.
Public relations refers to the notion of making use of mass media to communicate the word of the company to its audiences. Companies also have public relations managing influential media personnel in a way that makes it impossible not to want to carry the message of the company in their publications.