Here's a good overview of cloud types with photos of the clouds we're studying this week.
This site has a ton of great links, including NASA's cloud site for kids; cool clouds from space; how to make a water cycle wheel; and more.
Here's a link to some easy hands-on experiments from this brief, informative summary on clouds with more links to more pictures.
And here's a link with poetry, another science experiment, and a number of art projects related to clouds!
NASA's created a couple of videos for kids on this: a basic one on clouds and their types, and this one on shapes and formation of clouds, with a neat demonstration on making a cloud. This video also demonstrated the creation of clouds--less exciting, but pretty explanatory, and shows how you can make a cloud with a 2-liter bottle at home. Here's a very basic video on types of clouds for reading-aged children with no voice-over.
For older students, here are some wonderful worksheets that incorporate some math, too.
For memory purposes, the poem can loosely be formed to "The Farmer in the Dell." (Someday my creativity will think of a different song that has the same meter.)