Blog is short for Web Log. Generalizing about blogs is difficult because there are exceptions to everything, but the typical blog is a personal communication: It's the product of one person who is writing (or photographing or whatever else the content of the blog is) often as much for herself or himself as a diary as for others. Most blogs consist of frequent postings of brief items organized in reverse chronological order -- last in, first out. The typical blog interface presents abstracts of some or all the entries for the current month on the home page, a calendar UI for selecting entries in the current month by date, and a list of archived entries for previous months and years. Most blogs include categorization and search capabilities.
Blogs include their readers in the communication in several ways. Readers can usually post comments that appear associated with items in the blog -- comments that often amplify the content of an item by including a link to a related item in another blog. In fact, one of the most common purposes for a blog is to organize links to other Web content -- not only in the posted items, but in features like the blogroll, a list of the blogs that the blogger reads and recommends. To make it easy to link to a blog item, most have assigned permalinks, URLs that won't change if the item is moved in the blog structure or archived.
Many blogs also support trackback, a sort of comment protocol that lets a blogger create an item commenting on an item on another blog, then place a link to his comment as a trackback entry with the item on the other site.
Blogs can be read just like any other Web content by following a link to a Web page, but most of them are also accessible as XML-formatted feeds that are consumed by software called blog readers or blog aggregators. There are several formats for these feeds. The two most common are several versions of RSS and Atom. RSS, the most widely used, stands for Really Simple Syndication. It's built on an RDF Site Summary that uses the Resource Description Framework, a W3C standard for metadata. The blog software typically generates a new RDF document each time a new item is added. This document has a permalink that is accessed by blog readers that collect new entries from all the blogs they are subscribed to in a common interface.
Atom is both a syndication format and an application programming interface (API). It's intended audience is the blogger as well as the developer. For more on Atom, see the Blogger Web site.
Just as RSS feeds let a reader subscribe to a blog, there is an emerging standard: Outline Processor Markup Language or OPML that does the same for blogrolls.