What are three types of specifications used by content aggregators to distribute content?

A content aggregator is a business that gathers and organizes Web content then distributes, or feeds the content to subscribers for free of for a fee.

RSS 2.0 (Really Simple Syndication) is a specification that some content aggregators use to distribute content to subscribers.

Atom is another used to distribute content.

Some browsers use internet Explorer's WebSlices, that enables content aggregators to mark sections of their web pages as feeds to which users can subscribe.