A floppy drive is a machine that is designed to read floppy disks. Floppy disks are the removable devices that you stick in slots in the front of the machine. I know, they look square, not disk-shaped, and they don't look floppy at all, but they are indeed floppy disks. Floppy disks come in hard plastic cases to make them a little more sturdy and easier to handle, but inside the plastic case, there is an actual disk. It is made of a plastic-like substance called mylar which really is floppy. Floppy disks come in two main sizes; 5 1/4 inch, and 3 1/2 inch. The 5 1/4 disks are becoming obsolete, but you still see them from time to time. The 3 1/2 inch disks, although physically smaller, can usually hold more information!
There are different flavors of floppy disks. In a modern computer, the only kind of floppy you need to purchase is High Density 1.44 MB. As you can guess, they hold 1.44 MegaBytes of information, which is a pretty good amount. (You could hold several hundred pages of text or about a dozen full color pictures on a 1.44 MB disk) This type of disk may carry some other markings as well, such as DSHD or HD. The HD is the important part. That tells you it can handle 1.44 MB. If you have an older computer, you may find that it needs a different type or size of disks. Check your documentation to be sure.