There is no limit to the shapes or types of clouds you can create in Illustrator, and it only takes a few minutes. Whether you have never used Illustrator before or have been using it for years, this is one of the most simple objects to create. This technique can be used for other shapes as well.
Start with a main shape. Most clouds start with a large oval. Make sure your colour settings have an outline so that you can see what you are creating. The image in the example is white with a black outline.
Add many different ovals and circular shapes to this main shape. Keep the outline around them for now, so that you can see what shape you are creating. Think of this as a sculpture, and with each shape you add, the more distinct your cloud will be.
Select all of your shapes and turn off the outline. In your pathfinder palette (you can get to this palette by going to Window, and pathfinder), the first shape mode should be "Add to shape area." Click on this. Now all of your shapes are one big shape. You can put an outline on again, and you should be left with something very similar to the example image.
Using another colour for the outline can give your cloud more dimension--a light blue or brown are good colours for this.