Apart from this article, I also gave a talk at OHM2013 about this subject. As soon as the video recordings made at that time are available, I'll link there.
Hard disks: if you read this, it's pretty much certain you use one or more of the things. They're pretty simple: they basically present a bunch of 512-byte sectors, numbered by an increasing address, also known as the LBA or Logical Block Address. The PC the HD is connected to can read or write data to and from these sectors. Usually, a file system is used that abstracts all those sectors to files and folders.