This section tries to clarify some of the technical jargon used along with feed readers.
Feed readers or news aggregators can be realised as web applications or as desktop applications. The place of execution (your computer vs. some webserver) implies certain advantages and disadvantages.
A desktop news aggregator runs on your own PC and stores aggregated feeds locally. It accesses each news feed and downloads its newest headlines periodically. Note that not all desktop news aggregators can work offline. Some desktop aggregators retrieve feeds each time you access them instead of caching them for later use. Liferea as an offline capable desktop aggregator caches feeds and is suitable for use with portable devices (laptops, hand helds...) that do not have a permanent internet connection.
A online news aggregator (for example TheOldReader) is run on a remote webserver which you can access from everywhere as long as you have network access. When you log in to the online news aggregator it usually doesn't need to fetch the most recent headlines of you feed subscriptions because it implements feed caching to save bandwidth.
For a news aggregator a news feed is a distinct information source. A news aggregator usually retrieves news headlines from many news feed subscriptions. The term "news feed" is not very precise as feeds can also provide weblog postings, podcasts and practically every type of content. The same goes for the term "news headlines": often "article" or "post" would be a better description. Nonetheless the prefix "news" is usually used with the terms "feed", "headline" and "aggregator".
When you add a new feed then you have to specify the source of the feed, so that the news aggregator can initially retrieve it. The feed source is usually a HTTP URL. This URL with optional authentication and proxy settings is a "subscription" managed by Liferea.
To manage and easily navigate your subscriptions Liferea provides you with a hierarchic tree of your subscriptions. Similar to email clients the "feed list" is located in the left pane. You can organize your subscription after topics in different folders.
When a news aggregator updates a news feed it downloads the source document from the source URL of the feed subscription. This source document provides a set of the most recent headlines which then need to be merged against the current feed cache of the aggregator because the set of headlines provided by the feed source changes over time.
Headlines as provided by the feed source are the smallest information unit handled by aggregators.
In Liferea headlines are also called items and are presented in the so called item list. Following the email client paradigm the item list is often presented in the upper right pane in the news aggregator interface.