There are 3 types of disks that you may use. The most common is a hard drive. When a PC is put together, the hard drive is screwed onto the chassis of the PC so that it cannot move about freely, which could damage a hard drive. They are all about the size of a paperback book, but they can hold varying amounts of data. Another type of disk that you may use is a floppy disk.
Floppy disks are different from hard disks in that they are portable.
You can slide one into it's drive, copy files to it, remove it from it's drive, and take it home with you. Floppies make it easy to pirate software, but I would never do that. That would be irresponsible. Floppy disks generally hold less than 1.5Mb. Because they are portable, they are susceptible to damage, and therefore have shorter working lives.