Blogs are great tools for publishing content online.  They are not just for publishing personal thoughts, feelings, and ideas, however.  They have many different applications in todays Internet world. Here are a couple of examples:

Personal: A blog's primary role is a tool to publish an online diary or journal.  It is considered personal when the majority of blogging content is about your life.  It usually contains photos, events from your life, personal videos, and personal thoughts, opinions and feelings.  http://ma.tt/ is a good example of a personal blog.  It is of Matt Mullenweg, the creator of WordPress and founder of Automattic, the company behind WordPress.  He talks about what's going on in the world, but mostly as it pertains to him.  He's not above just posting a photo of his dinner right before he dives in.

Business: Business users can use blogs to promote their company and companies product or service offerings.  It is utilized as more of a marketing and promotion tool.  They usually focus on useful information for the end user and a call to action usually at the bottom of the post.

Media: News sites such as Fox or CNN utilize blogs to publish up to date and relevant content to it's readers.  These companies usually blog about the information in their sector and it's not uncommon for a single news source to have multiple blogs in a single website.  They are each usually run by a particular blogger.

Government: Big brother has also hopped on the blogging bandwagon and it is now commonplace to see US senators publish related info about what's going on in the world of politics as it relates to them and their party.

Professional: Basically a professional blogger is like a freelance writer writing about a certain topic or industry.  They publish their latest on their blog so similar interested people can get the latest news about that particular topic.

Whatever the type, make sure you clearly define what type of blog you want to create early on.  It is better have a clearly defined blog that has little content than a blog that can't decide what it wants to be.