Many blogs and websites provide content via RSS (Really Simple Syndication) feeds, which enable the content from the site to be subscribed to by a user and collected, sorted and read through a tool called an aggregator. This makes it possible for a user to read a collection of blog entries and/or website updates all in one place, using one application. Depending on the purpose of the aggregator, it may go by a different name. For example, "feed readers" or "blog readers" are aggregators designed for collecting news feeds and blogs.
For a quick overview of RSS feeds and how they work, visit this post from the CIT blog or view the original link.
Blog/feed readers

Though many modern web-browsers have built-in RSS feed readers (Firefox, Internet Explorer and Safari all do), they require the user to always access his or her collection of feeds on the same computer, using only that browser. For this reason, many people prefer to use an online/web-based feed reader which can be accessed using virtually any online terminal, in some cases even using mobile phones and other devices.
Homepage portals
What if a user wants to access all of his or her blog and website feeds in one place, plus photos collected in an application like Flickr, or favorite videos in an application like YouTube? Homepage portals aim to create a virtual, web-based "one-stop" access point for users, where a collection of links, mini-applications (such as daily weather animations and calendar reminders), and feeds from blogs and image/video sites all come together.
Podcast/Media Aggregators
Other aggregators specialize in handling podcasts and rich-media feeds. CIT has more information available on one of the most popular podcast aggregation tools, iTunes as well as iTunesU.
Short video: RSS in Plain English, from Common Craft
This TechCrunch article provides a good overview and evaluation of most current blog readers
The Newbie's Guide to Google Reader from Webware
Several advanced and useful tips for using Google Reader from Lifehacker
Lifehacker also has a nice article explaining the key differences between desktop-based RSS readers and web-based readers