Drives are divided into sectors. Each sector contains a fixed number of bytes, eg 4096 bytes.
File systems can be partitioned using either MBR or GPT:
Master Boot Record (MBR)
Globally Unique IDentifiers (GUID) Partition Table (GPT)
GPT is newer than MBR.
The first sector of a disk is the boot sector. This applies to both MBR and GPT.
GPT does not use the boot sector, and it is just kept for compatibility reasons. An EFI system partition is used instead, as discussed later.