a.k.a. recommender system, recommend platform, information filtering system
A recommendation engine is a system that predicts the "rating" or "preference" that a user would give to an item (such as music, books, or movies) or social element (e.g. people or groups) they had not yet considered, using a model built from the characteristics of an item (content-based approaches) or the user's social environment (collaborative filtering approaches).
Some Web sites that use recommendation engines include:
Amazon.com - when viewing a product, the store will recommend additional items based on a matrix of what
other shoppers bought along with the currently selected item.
Pandora Radio - takes an initial input of a song or musician and plays music with
similar characteristics, based on a series of keywords attributed to the
inputted artist or piece of music.
Netflix - offers predictions of movies that a user might like to watch based on the user's previous ratings and watching habits (as compared to the behavior of other users), also taking into account the characteristics
(such as the genre) of the film.