From iLearn Technology:

What it is: Glogster is a great creativity site who’s tag line is “poster yourself”. A ‘glog’ is basically an online poster web page. Students can combine text, pictures, graphics, video, and audio to create an interactive online poster. Glogster has a very simple to use interface. The final glog can be hosted by Glogster or you can embed it into a wiki, blog, or class web site.
How to integrate Glogster into the classroom: Glogster is an awesome way for your students to display knowledge. Instead of creating a poster for a presentation, students can create an interactive glog to display information. Glogster can be used for history, math, language arts, book reports, science, social studies, and for public service announcements. In fact, I am having a hard time coming up with a subject that couldn’t use Glogster in some capacity. Students can create these online posters to display any knowledge or learning. You really have to see this site, the creativity that it allows for will get your wheels spinning. Once you start using Glogster with your students, you are going to think of all kinds of new applications. Because Glogster has the ability to handle audio, students can create podcasts (using Audacity, Garageband, G-Cast or Gabcast) and upload the content into their glog. So cool! I really love that students can share their school work and accomplishments online with classmates, family, and friends. Give your students an authentic audience and their work will dazzle you!

Tips: You can check out a quick sample glog that I created here about Internet Safety.

Also, here's a great blog post from Tina Coffey on how teachers are using Glogster in the classroom.