They vary in their content, mine for example is just like a diary that i write for myself and others to read. Sometimes I post stories from my daily life along with pictures and audio clips that are applicable.
Lately the blog movement has become a real force as a means of alternative media. There are litteraly thousands of sites delivering proprietary editorial content on subjects from local and international news, to entertainment, fashion, technology or business. Famous examples would be the drudge report which broke the monica lewinsky scandal before any conventional media outlet, or smokinggun.com which follows the la police beat looking for celeb scandal.
If there is a topic you are interested in, odds are there is a blog or blogring (group of blogs syndicated in a cohort) on that subject.