Blog is the condensed name for a 'weblog' or 'web log'. Blogging is the action you perform when you make a post to a web log or blog, comment on a previous blog post or maintain a blog. Many people who make posts to a blog on a regular basis are referred to as bloggers.
Basically, a blog is an online journal where you can post your thoughts, observations and comments about a particular topic that the weblog covers. A blog is just a simple web page that has chunks of information called posts added to them on a regular basis. Depending on how it was setup by the person maintaining the blog, the posts can be all on one page forcing visitors to scroll down, or each post will have its own individual web page. When you are blogging and visiting different blogs, you will see the most recent posts first at the top of the web page.