So, for description of a partition it is necessary to specify the beginning of a partition and its length in blocks. A hard disc can contain 4 primary partitions. In the beginning of this sector there is a boot code which reads the partition table and defines an active partition. And then the sequence of operations repeats. I.e. it loads a boot sector of this partition according to the same address and passes over the control.
Partitions contain a file system which is a system of block marking for file storage.
After creation of a file system on a partition and after files of an operating system are placed here, the partition becomes a boot one. A boot partition has in its first block a small program for booting of an operating system. But for booting of a definite system it is necessary to start its boot program from the first block. A bit later I will tell you about it in depth. Partitions with file systems mustn't overlap, because different file systems have a different idea on how files are to be placed, but when files are put on the same physical disc space, the file systems conflict.