Drives with removable media will be accessible from the front. This includes the two sizes of floppy drives, tape drives, CD-ROM drives, and other newer kinds like Zip drives.
Each drive will have a small LED light that will light up when that drive is being accessed. These are in green on the diagram.
You probably don't have all of these different kinds of drives. Notice that only a certain number will fit in the spaces provided in your case. There are external versions, too, that plug into the back of the computer. These are handy if you don't have room for another device or if you want to use the device on more than one computer. The internal types are usually faster though.
Sound is an important diagnostic tool for these drives. Pay attention to what your drives sound like under normal conditions so you'll be able to catch problems right away.
A badly behaving drive can ruin the media you put in. If you suspect something is going wrong with a drive, don't use media with important data on it for testing. Use either a blank or something with unimportant data until you can be confident that the drive won't trash it!