Hard disks are usually partitioned. That means they are divided into independent sections called partitions or slices. The operating system treats each partition as if it was a separate device. Thus partitions can be used for different purposes and can have different formats.
Unfortunately, partitions don't come out of nowhere. Their locations must be recorded on the disk itself, usually in the first few sectors. Naturally, there are just as many such partitioning schemes as there are vendors. The data that records partitions is called the partition map, partition table, disklabel, or VTOC (for virtual table of contents).
At least on the PC there is something of a standard partitioning scheme, originally defined by MS-DOS. Some PC Unix systems keep using their own scheme, but put it inside a PC-style partition, to allow other operating systems to co-exist on the disk.