A typical blog has a main page and nothing else. On the main page, there is a set of entries. Each entry is a little text blurb that may contain embedded links out to other sites, news stories, etc. When the author adds a new entry, it goes at the top, pushing all the older entries down. This blog also has a right sidebar that contains additional permanent links to other sites and stories. The author might update the sidebar weekly or monthly.
Basically, a blog is a lot like an online journal or diary. The author can talk about anything and everything. Many blogs are full of interesting links that the author has found. Blogs often contain stories or little snippets of information that are interesting to the author.
Even though blogs can be completely free-form, many blogs have a focus. For example, if a blogger is interested in technology, the blogger might go to the Consumer Electronics Show and post entries of the things he/she sees there. If a blogger is interested in a certain disease, he/she might post every news article and every piece of research he/she finds on the disease. If a blogger is interested in economic issues, he/she might post links to articles that discuss the economy and then offer commentary on them.
There are people who use their blogs simply as a scrapbook -- a form of online memory. Whenever the author finds a link or a snippet of information that he/she wants to remember, it gets posted in the blog. Even if no one else ever looks at it, it is still useful to the author because the blog is a searchable electronic medium that the author can access with a Web browser anywhere in the world.
In other words, a blog can be anything the author wants it to be. The thing that all blogs have in common is the reverse-chronological ordering of entries.