Every file system has some place to store global information like the size, volume name, and anchor pointers to other structures. This place is usually one special sector or block at the beginning of the device. Almost all file systems include a certain constant value somewhere with that data, dubbed a magic value (or string). This is what disktype uses to identify file system formats.
Some file systems use several such structures, or store backup copies at other locations inside the file system. Names are likewise variable. The Unix world uses superblock almost exclusively. Other terms include boot sector, root sector, root block, volume descriptor and volume header.
Many file systems don't put the superblock at the very start of the device. The space before the superblock can be used for boot code or partition maps. In some cases, one other reason for the offset is increased resilience against DOS formatting tools.