A blog (short for weblog) is a website usually managed by one person (but not always) that journals a persons thoughts and ideas about a particular subject. Many blogs are personal diaries detailing someone's experiences.
You can contrast a blog with a forum, where the blogger really controls the content of the postings. Visitors to blogs can post comments, but a blog is less of a public forum. Visitors typically visit blogs to read the latest news and opinions of the blogger.