Specifically, teachers or students who sometimes turn in their ipads at some point and then have a new iPad issued to them later should use iCloud. Back up your old iPad and then turn it in. When you get your new iPad, you can restore all your apps and settings on the new iPad. Your new iPad will look and act much like your old one.
This will also work if you own your own iPad at home, but then you are issued another one through school. You can "copy" your personal iPad's data, apps, and settings to your school-issued iPad very easily.
What exactly gets backed up? Photos, videos, and all of your purchased apps are among the most important things. There are others, of course, but these are the big three. The entire list of things that get backed up can be found on a different Apple page called "iCloud Backup."
Photos and videos take up a lot of space. You could easily exceed your free 5GB of space in photos alone. If you exceed your free 5GB you only have two viable options: move your data to another location like Google or Dropbox which offer much more free storage than Apple, or you can buy more space through Apple's iCloud service.
Do you need directions on how to back up your iPad? Read Apple's KB article called "iOS: How to back up and restore your content." You can find the backup feature by going to Settings -> iCloud -> Storage & Backup on your iPad 2, 3, or 4.
Advanced users might be using iTunes Backup instead of iCloud. Some of you might be using BOTH at the same time. If you want to learn which one is best, or if you are interested in why you would use both at the same time, read the Apple article called, "Choosing an iOS backup method (Should I use iTunes or iCloud to back up my iOS device?)."