In its broadest definition, a recommender system (RS) is a technique that's used for providing suggestions for items that are useful to a person. These suggestions are meant to help in various decision-making processes, usually related to buying or consuming a certain category of products or services. They can be about buying a book, listening to a song, watching a movie, eating out at a certain restaurant, reading a news article, or picking the hotel for your next holiday.
People have relied on recommendations pretty much since the beginning of history. Some RS researchers talk about the first recommendations as being the first orally transmitted information about dangerous plants, animals, or places. Others think that recommendations systems functioned even before language, by simply observing the effects on other humans of consuming plants...