My understanding is that the mandala shows the order of the universe as a Buddhist sees it. It also is repetitive and round, reinforcing the key Buddhist concepts like meditation (try making one and see if you don't zone out a little). Mandala's are also round and a reminder of all the round concepts in Buddhism (the wheel of law, the cycle of Karma, the cycle of suffering that is life). Mandalas can be made out of anything.
Here's a fruit mandala.
Here's one made of chalk.
Can you figure out what this one is made of? (I'm super jealous of that color coordinated collection BTW!)
Many traditional Buddhist Mandalas are made of SAND. Yep, its hard to work with, but life is suffering so that's kind of the point! Below is a GREAT video showing Buddhist monks making a sand mandala. The video is stop motion because they usually spend DAYS making these things and it has some information along the way. Plus it has a catchy tune and you can dance to it. (Just kidding) Seriously, click the link. You know you want to! If your in my class,post a comment about why you think they end the sand mandala the way that they do (is there any symbolic meaning there?) and I may throw an extra credit point your way.