Aggregator (also referred to as: RSS aggregator, news aggregator, feed reader) is a software that gathers information published by different sources and organises it together under a common interface.
The main idea behind aggregators is saving your precious time. Instead of having to go and visit each blog you like, you can subscribe to its feed, and let your aggregator do the rest for you.
The way aggregators work is that you add a list of blog feeds to your aggregator, and it will check each of them for the fresh content. Most feed readers will have an interface very similar to your favorite email program – you'll have an inbox with a number of new messages – posts from various blogs which you haven't read yet.
There are two types of feed readers: online and offline ones.
Online ones are represented by a large number of websites for your (mostly free) use. You register yourself an account, and start adding links to feeds from various blogs.
Online feed aggregators (just to name a few):
There are also many offline feed aggregators, which are present in various forms like stand-alone applications and extensions to browsers and email clients. It is also increasingly popular to have feeds aggregation built-in into a web-browser.
Offline feed readers: