Marketing is an umbrella term for all promotional efforts a publisher makes on behalf of a book. In contexts where marketing and publicity are framed as two distinct categories, the simplest way to make the distinction is that marketing refers to promotional efforts the publisher pays for and (mostly) controls; publicity refers to strategies to encourage other people, organizations, and outlets to use their resources to promote the book.
Marketing includes all promotional opportunities that the press spends its own time and money on. Social media promotion, print and digital ads, conference displays, and outreach to sales reps are all marketing efforts. Publicity also requires a lot of effort from the publisher’s staff, but its outcomes (reviews, podcast interviews, excerpts, best-of lists, etc.) are outside of the publisher’s direct control. While marketing and publicity staff have slightly different purviews—and the exact responsibilities of each group will vary from press to press—they work closely together to ensure that their efforts are supporting and complementing each other.