You need a bootloader to boot into linux. You can choose:
MBR - Recommended if you have no other bootloader to load linux with. Will usually add windows boot option as well.
Root partition - Can't be used by itself, ie. needs another bootloader that points to it.
Floppy - Writes the bootloader to a floppy, you can then boot to linux with having the floppy inserted.
Skip - You have no bootloader and need another one capable of booting linux. You then need to add all parameters to the loader yourself.