I like the definition of a feed given by good old Wikipedia. It is as follows:
A web feed is a data format used for serving users frequently updated content. Content distributors syndicate a web feed, thereby allowing users to subscribe to it. Making a collection of web feeds accessible in one spot is known as aggregation, which is performed by an Internet aggregator.
In the typical scenario of using web feeds, a content provider publishes a feed link on their site which end users can register with an aggregator program (also called a feed reader or a news reader) running on their own machines; doing this is usually as simple as dragging the link from the web browser to the aggregator. When instructed, the aggregator asks all the servers in its feed list if they have new content; if so, the aggregator either makes a note of the new content or downloads it. Aggregators can be scheduled to check for new content periodically.
The kinds of content delivered by a web feed are typically HTML (webpage content) or links to webpages and other kinds of digital media. Often when websites provide web feeds to notify users of content updates, they only include summaries in the web feed rather than the full content itself.
So please be sure to follow the links in the definition to get even more definitions.
Part 3 will deal with some popular RSS FEEDS