The bootloader is a program that's responsible for booting the system. Most of the time, it will have a menu to choose an option from, making it a boot manager.
A bootloader is installed onto the boot partition of the computer during a distribution's installation process. Most of the times, boot partitions may need from 200-600MB of space to work properly. Most bootloaders also have theming capabilities, take, for example, the previously mentioned GRUB.