Cosmos 4 kids
Cosmos4 Kids: Galaxy Cluster Basics
Understand what galaxy clusters are on this site which contains both information and a video.
PBS
Pbs: Nova Online: Galaxies, Clusters, and Superclusters
PBS site explores these building blocks of the universe as well as their various types and clusters.
Other
Sky Server: About Astronomy
This site from SkyServer provides the history of astronomy as well as links to mapping the sky, galaxies, cosmic structures and much more. Several pictures are given, and the article is medium in length.