I found this article by Kristi Hines and thought is was a great definition of what a blog is and how it can be used.
A blog is a website, or section of the website, consisting of regularly updated entries, in the form of news or informational articles. These entries, known as blog posts, are displayed on the homepage or a blog page within the site. The homepage of a blog or a blog page will show a short summary or the beginning portion of five to ten blog posts. Visitors can click the titles or a read more link to read the full entries.
Individuals, businesses, and organizations alike create blogs. The goal of a blog is typically to share stories, entertain, educate, or promote the sale of products and services. Blog content can include text, images, video, and audio.
Blog post content is created by one or more individuals, or bloggers, for the website. Bloggers can include the blog owner, employees of a business, outside guest contributors, and freelance writers.
Most blogs give readers the ability to comment on each blog post entry, although some in recent years have turned off discussions due to spamming.The absence of blog comments does not necessarily mean that the website is not a blog, but the presence of blog comments almost assures that it is a blog.