There are two types of 3.5-inch drive bays: internal and external. Internal 3.5-inch drive bays hold drives, like hard drives, that do not need to be touched from outside of the computer. External 3.5-inch drive bays hold drives, like floppy drives, that need their disks to be manually inserted from outside of the computer. The picture above shows a Zip drive on the left and a floppy drive on the right. Below is a picture of a hard drive.
Hard drives are sealed and contain disks that are not removable. For this reason, they can be hidden inside of the computer, never to be seen. This is why most hard drives are internal bay drives. The hard drive is where your files and programs are stored for everyday use. They are always there unless someone or something erases them. Normally, computers will have one or two external, and two or three internal 3.5-inch drive bays.