This hard drive on formatting consists of various tracks and sectors. Refer to the figure above. At the beginning of the hard drive, we have the boot sector. Rest of the area on the hard drive is for the purpose of storing data. Hard drives are generally broken into primary and extended partitions.
Primary partitions are bootable partitions and can be made to boot through any boot loader.
Many BIOS' prevent booting off logical partitions. Linux has the capability to support a maximum of four primary partitions. If additional partitions are required, one of the partitions can be made an extended partition where additional logical partitions created. One important point to note is that the partition tables of some logical partitions are not accessible as the first block of some devices, while in the main partition table all boot sectors and partitions tables of the extended partitions are. Partition tables are generally stored in the boot sector of the partition and normally only the partition boot sector of the entire disk is usable as the boot sector.