What is Syndication?
Syndication is a user-initiated subscription service where content from web pages is pushed (sort of) to your computer. You can think of it as turning your favorite web pages into newsletters that are sent to you whenever new articles are published. Instead of checking cnn.com every ten minutes for new technology articles, those articles are sent automatically. You can control how often the website is checked for new content. You can also sometimes control what content is sent to you as with the technology example above from CNN.
RSS
You will hear the acronym RSS thrown around a lot when syndication is mentioned. This is a specific format of a syndication feed.
Agregator
This is a tool that collects RSS feeds from different sites for you to read. It is very similiar to email software. It will keep track of what articles you have read. Many of the better web browsers and email clients are now adding RSS support.
More Information
Wikipedia article on RSS