An information filtering system is a system that removes redundant or unwanted information from an information stream using (semi)automated or computerized methods prior to presentation to a human user. Its main goal is the management of the information overload and increment of the semantic signal-to-noise ratio. To do this the user's profile is compared to some reference characteristics. These characteristics may originate from the information item (the content-based approach) or the user's social environment (the collaborative filtering approach).

Whereas in information transmission signal processing filters are used against syntax-disrupting noise on the bit-level, the methods employed in information filtering act on the semantic level.

The range of machine methods employed builds on the same principles as those for information extraction. A notable application can be found in the field of email spam filters. Thus, it is not only the information explosion that necessitates some form of filters, but also inadvertently or maliciously introduced pseudo-information.

On the presentation level, information filtering takes the form of user-preferences-based newsfeeds, etc.

Recommender systems and content discovery platforms are active information filtering systems that attempt to present to the user information items (film, television, music, books, news, web pages) the user is interested in. These systems add information items to the information flowing towards the user, as opposed to removing information items from the information flow towards the user. Recommender systems typically use collaborative filtering approaches or a combination of the collaborative filtering and content-based filtering approaches, although content-based recommender systems do exist.

Before the advent of the Internet, there are already several methods of filtering information; for instance, governments may control and restrict the flow of information in a given country by means of formal or informal censorship.

Another example of information filtering is the work done by newspaper editors and journalists who provide a service that selects the most valuable information for their clients i.e readers of books, magazines, newspapers, radio listeners and TV viewers. This filtering operation is also present in schools and universities where there is a selection of information to provide assistance based on academic criteria to customers of this service, the students. With the advent of the Internet it is possible for anyone to publish anything they wish at a low-cost. Because of this, the quantity of less useful information has increased considerably and consequently quality of information has improved/disseminated. Due to this problem, work to devise information filtering to obtain the information required for each specific topic easily and efficiently began.

A filtering system of this style consists of several tools that help people find the most valuable information, so the limited time you can dedicate to read / listen / view, is correctly directed to the most interesting and valuable documents. These filters are also used to organize and structure information in a correct and understandable way, in addition to group messages on the mail addressed. These filters are essential in the results obtained of the search engines on the Internet. The functions of filtering improves every day to get downloading Web documents and more efficient messages.