Advertising and publicity are two very different communication tools, even though both employ the mass media as a vehicle for reaching large audiences.
Traditionally, most marketers placed heavy reliance on advertising and only occasionally used publicity.
On the other hand, public relations practitioners have primarily relied on publicity--or, as they sometimes prefer to call it, media relations--and only rarely used advertising.
This does not mean that advertising should be seen only as a marketing tool and that publicity should be seen only as a public relations tool. Thoughtfully used, both tools are valuable for both functions.