A blog (or "web log") "is a web-based publication consisting primarily of periodic articles (normally in reverse chronological order)" - Wikipedia.
A blog is your own online journal. You can write about your opinions, your experiences or just whatever you would want to share with the world.
To start posting to your blog, click on the "my blog" link on your navigation block (the one with your user name on the left).
Fill in a title for your post and in the "body" section, enter your blog! When you are finished typing, click the preview button at the end of the page, then check over your entry and finally click on submit at the end of the page. Others will be able to read your thoughts and post comments on them.