BIOS boot partition

a partition on a data storage device that GNU GRUB uses on legacy BIOS-based personal computers in order to boot an operating system,
when the actual boot device contains a GUID Partition Table (GPT). Such a layout is sometimes referred to as BIOS/GPT boot.

On MBR drives the boot loaders often cheat a bit by storing part of them in the next sectors called "MBR gap", "boot track", or "embedding area" which are often leaved empty by disk partitioning tools. On a GPT disk the sectors right after the MBR are GPT data structures, hence can't be used for that purpose and you need to create a small BIOS Boot Partition for Grub to store its data

On a BIOS/GPT configuration, a BIOS boot partition is required. GRUB embeds its core.img into this partition.

For UEFI systems this extra partition is not required, since no embedding of boot sectors takes place in that case.
However, UEFI systems still require an EFI system partition.

BIOS-Boot partition:

    Mount point: none
    Type: no filesystem
    Description: the BIOS-boot partition contains GRUB 2's core. It is necessary if you install Ubuntu on a GPT disk, and if the firmware (BIOS) is set up in Legacy (not EFI) mode. It must be located at the start of a GPT disk, and have a "bios_grub" flag.
    Size: 2MB.

EFI partition:

Mount point: /boot/efi (no need to set up this mount point as the installer will do it automatically)
    Type: FAT (generally FAT32)
    Description: the EFI partition (also called ESP) contains some boot files. It is necessary if the firmware (BIOS) is set up to boot the HDD in EFI mode (which is default on more and more modern, > year 2011 computers). It must be located at the start of a GPT disk, and have a "boot" flag.
    Size: 100~250MB