Think of your bootloader as analogous to the bios on your PC. (just analogous to, but not identical)
Normal function is to manage the system memory layout and to boot the Android OS.
Android is a compact Linux plus the Dalvik VM (virtual machine) and system applications that run inside the Dalvik VM (as will all of your apps).
The Linux operating system, like all operating systems you know (including Mac or Windows) consist of task management, services and a kernel.
The kernel is the foundation that ties services to the hardware. (It's ok to think of device drivers as glue between services and the kernel.)
Android (Linux (including the kernel) plus Dalvik VM plus apps) is contained in a payload that we call a ROM (that's slang because it's not really read-only memory).
All ROMs include a kernel, and often a replacement kernel can be installed separately provided it is compatible with the rest of the ROM.
Hope this helps!