A publisher is a company that takes responsibility for marketing and distributing a software product.
In terms of gaming, during the 1980's and 1990's, it was typical for a team of individuals to develop a game, and then enter negotiations with a publisher over marketing and distribution of the game, in exchange for a share of revenue.
However, in the last decade, it has become very common practice for publishers to seek out, and take over, smaller gaming companies, using their own corporate brand on games developed by members of the incorporated company, in many cases without the old developer's name surviving the transition. One notable example of this was Electronic Arts taking over Westwood Studios; EA expunged the Westwood name from future software titles, and games were attributed to the "EA Games" brand of Electronic Arts.
Many illustrious companies who created games that redefined their respective genres have either folded, or been absorbed into greater corporate entities; DMA Design Ltd, famous for the Grand Theft Auto franchise, was bought initially by Gremlin Interactive, who were later acquired by Infogrames. Take-Two Interactive later bought DMA Design outright, renaming it Rockstar North.