A sideblog is meant to illustrate your immediate thoughts, movements or status update, and is usually less than 200 characters.
Where a blog post may be compared to a newspaper opinion piece, a sideblog would be akin to the "news in brief" column.
Most sideblogs are actually embedded micro-blogs that appear in a small sidebar box next to the main blog posts and use feeds or plugins to display the content.
Frequently updating one's status and frame of mind in an online platform to be viewed by friends gained widespread adoption through social networking tools like Facebook and MySpace.
While those tools are still largely desktop-bound, micro-blogs (and, by extension, sideblogs) are designed to be updated from mobile devices as well as desktop computers.