Publishers are the companies who buy, contract, amass, licence and sell intellectual products. They work according to what Habermas called a factory model of culture, where culture is a finished 'product' to be sold to 'consumers', instead of something made for its own sake. Later they also included record-labels, studios and private research and development companies. They are often referred to as authors in jurisprudence, though they have quite different, and changing interests.
For example in the 16th century the Stationers held a monopoly on publishing, and used this bargaining position against authors. And in the 18th and 19th centuries American publishers were opposed to copyrights while many authors were in favour of them. Also most big movie-studios are in Hollywood now, and not along the East Coast where they started, because in 1909 they fled from New York to escape the enforcement of patents on filming-equipment. While nowadays Disney, Warner-brothers and other large publishers built their business-models around IP, and lobbied for an extension of copyright-terms long beyond the lifetime of individual authors.
Also as a group they now have their own interests. First of all corporations can exist for much longer than humans, thus they are expected to be interested the mentioned term-extensions. They depend on IP for their stock-value and thus their existence. They also need profits, just like artists need an income, but a difference is that for musical artists most of their income comes from concerts, not from CD-sales, while this is the other way around for record-labels. Most notably publishers are middle-men between the author and the public, and they naturally are interested in maintaining this position.