Their disadvantage is that they don't hold very much info. The name comes from the flexible disk of magnetic particle-coated plastic inside the outside protective cover. They have read and write capability. A CD ROM holds about 650 MB of data and is removable. The heads use lasers to read the pits in the plastic disks.
Disk controller is an electronic board in the computer that controls the disk.
It is not inherently connected to anything. The computer only responds to keyboard if the CPU is looking at the keyboard to see if someone had pressed a key. If the CPU isn't looking, the person can hit the keyboard all they want and nothing will happen. It translates the data in the video memory into symbols on the monitor. It can thus draw letters and graphs on the screen.