"Blog" comes from "web log
". The first blogs were conceived as online diaries. People posted short articles about their lives or activities, often on a daily basis like a diary. Nowadays, there are millions of blogs and millions of bloggers. Some are still along the lines of the original diary or log concept; others focus narrowly on a single topic; some are highly technical; some are rambling; a blog can be almost anything these days.
The one thing most still have in common is that articles are indexed, at least initially, by the date published. As articles accumulate, older ones may be stored in an "archive" along with additional indexing schemes such as by topic "tags". I called my set of articles a blog so people would know to expect a set of possibly unrelated articles on a variety of topics, organized by date published. It allowed me to create a place for articles without knowing in advance what subject categories I'd have to allow for in the future.
There are even programs (such as one called WordPress) that supposedly allow the writer to focus only on writing and not on the technical aspects of getting their writings onto web pages, and they add features like the ability for readers to comment and discuss (such as what you expected to find), or for one article to reference an article on another site and automatically notify the other site that a reference has been created. But all these features add security problems, and there must be hundreds of thousands of hacked blogs. That's why I kept my blog as "plain" web pages, and put the comments and discussion features here in the forum.
That's the short version. For encyclopedic detail(!), see http://en.wikipedia.org/wiki/Blog