An aggregator, also known as a "feed reader," "RSS reader," or "feed aggregator," is an online program or client software that collates syndicated web content (referred to as "feed"), like blogs, podcasts, and vlogs from various sources into a single location for convenience, in effect creating a unique information space or "personal newspaper."

Through the use of aggregators, time and effort needed to regularly check websites for updates is reduced, as the aggregator itself will check the sources for updates at user-determined intervals. Aggregators are a common feature of portal sites, modern internet browsers and email programs.

A single browser display or web application is used for a consolidated view of all the syndicated content coming from various sources, which are usually supplied in the form of RSS, RDF/XML or Atom. There are also more advanced methods of aggregating feeds, such as AJAX coding techniques and XML components known as "widgets." The syndicated content that is aggregated is described as being "pulled" to the subscriber, in contrast with content "pushed" through email and instant messaging. It is much easier for a user to unsubscribe from "pulled" material than it is for "pushed." Unwanted "pushed" content is also referred to as "spam."

There are two types of aggregators: web-based and client software.

Web-based aggregators are online applications available through the internet and made accessible anywhere to a user with an internet connection. Under web-based aggregators, there are two subsets: personal web-based aggregators, which display user-defined content; and planet sites, which aggregate community blogs in a centralized location. Examples of personal web-based aggregators include My Yahoo, Pageflakes, Google Reader and Bloglines. Most examples of planet sites, such as Planet Planet and Planet Parrot, are prefixed with the word "planet," as they are named after the original Planet aggregator.

Client software aggregators are applications installed by users. They collate web feed subscriptions into a single application with a user-friendly interface, often resembling the three-panel composition (subscriptions are grouped in the left-hand frame, individual entries on the right) of popular email clients. They may also appear as news tickers which scroll feeds like ticker tape, or as plug-ins or extensions which are able to integrate feeds into the operating system or software applications.

As an effect of the number of feed subscriptions a user has, the volume of articles in aggregators may be overwhelming at times. To address this issue, several feed readers now allow users to categorize each feed into easily navigable categories by tagging the feed with keywords.