A long time ago, when the web was young, to run a personal web site you needed to edit every individual page. This was done either directly in the language of the web page (html) or using a wysiwyg editor. These pages needed individually uploading to the web server, and the links between them needed manually maintaining.

The blog came about to make this easier for quickly posting new entries. The main features of a blog are themed pages, on-web editing, a front page listing recent posts and a detailed page holding a more complete entry.

Most systems support comments, image posting, rss feeds and lots of other goodies under the hood.

Wordpress is the mot popular and can do most things out of the box. Drupal is another system I use, which can be configured as a blog, amongst other things.