"A weblog (usually shortened to blog, but occasianlly speeled web log) is a web-based publication consisting primarily of periodic articles (normally in reverse chronological order). Although most early weblogs were manually updated, tools to automate the maintenance of such sites made them accessible to a much larger population, and the use of some sort of browser-based software is now a typical aspect of 'blogging'. "
For me, a weblog or blog is a website designed to be able to update regularly and it is similar to a personal journal or diary, expect that the contents are viewable for public. Weblog often implement using software or public host server with pre-install weblog application. Weblog contain articles or entries that are grouped primarily by the date and time when it is posted.
They are people do make some distiction between an online journal and a weblog, where a journal tends to be author centric and a weblog tends to be the author's exprerience, observation and a 'log' of what he or she want to tell.
However, whether someone is either 'journaling' or 'blogging' is actually totally up to the author. This is because there are no rules being set for blogging.