A blog lets you easily get information out to your students and allows them to post comments about what you wrote. For instance you could publish a students poem and classmates could come in and comment on the poem, telling them one thing they like or dislike about the writing.
Two places you can create a blog that is safe for students would be blogger.com and edublogs.org - both seem to be ad free. You have a 100MB limit, but I don't think that will be a problem for most of us.
Another good web site for blogging is wordpress.com
At this web site is a quick little overview of what a blog can be: http://www.blogger.com/tour_start.g
If you use blogger.com you'll need a Google account. You can easily create one and then use the Google Reader application to keep notified of when comments are made to your blog so you can go and check it out.
Edublogs.org is a nice site and it is designed specifically for educators. This may be a better place to start off initially, but it doesn't have as many features as blogger.com. Both are really good places to start.
Click here to go to Edublogs.org (make sure to type in the correct address, or you'll be sent to other companies who offer blogging, but it's not as good)
Click here to go to Blogger.com (also know as blogspot.com, so don't be confused when your web site is called blogspot.com, blogger.com is the content creator site and blogspot is the content publishing site.)
Click here to go to Wordpress.com