On Linux, the hard disk. This command, dd, copies data in bulk from /dev/random (a special device on *nix that outputs random bytes) to /dev/sda (your hard disk). That means it starts to overwrite your disk with trash, rendering the system unusable.
The device file applications can use to access the first hard drive on most Linux systems. It provides raw access without filesystems or partitions.
/dev/sda1 is the first partition on that drive, /dev/sda2 the second, and so on. /dev/sdb is the next hard drive, /dev/sdc the next after that; beyond /dev/sdz, the naming scheme is apparently dependent on the hardware driver in use: Going from /dev/sdz to /dev/sdaa is what happens in the default SATA and SCSI drives, up to /dev/sdzzz, at which point you apparently run into problems. [1]
