There are two basic architectures for a recommendation system:

1. Content-Based systems focus on properties of items. Similarity of items is determined by measuring the similarity in their properties.

2. Collaborative-Filtering systems focus on the relationship between users and items. Similarity of items is determined by the similarity of the ratings of those items by the users who have rated both items.

In a content-based system, we must construct for each item a profile, which is a record or collection of records representing important characteristics of that item. In simple cases, the profile consists of some characteristics of the item that are easily discovered. For example, consider the features of a movie that might be relevant to a recommendation system.

1. The set of actors of the movie. Some viewers prefer movies with their favorite actors.
2. The director. Some viewers have a preference for the work of certain directors.
3. The year in which the movie was made. Some viewers prefer old movies; others watch only the latest releases.
4. The genre or general type of movie. Some viewers like only comedies, others dramas or romances.

There are many other features of movies that could be used as well. Except for the last, genre, the information is readily available from descriptions of movies. Genre is a vaguer concept. However, movie reviews generally assign a genre from a set of commonly used terms. For example the Internet Movie Database (IMDB) assigns a genre or genres to every movie.

Many other classes of items also allow us to obtain features from available data, even if that data must at some point be entered by hand. For instance, products often have descriptions written by the manufacturer, giving features relevant to that class of product (e.g., the screen size and cabinet color for a TV). Books have descriptions similar to those for movies, so we can obtain features such as author, year of publication, and genre. Music products such as CD’s and MP3 downloads have available features such as artist, composer, and genre.