The bootloader is vendor AND model-specific. It is not consistent across Android versions.
A common way to get into the bootloader (also called "recovery mode") is to hold a volume button down and reboot the device, releasing the button when you get to the bootloader. On my Samsung Galaxy S Vibrant, the easiest way is to hold both volume buttons plus the power button, releasing the power button once the phone reboots and you see the "Vibrant" logo, and releasing the volume buttons after the bootloader comes up.
On my Vibrant, the options include doing a factory reset and installing "update.zip" from the root of the internal SD storage. Your mileage may vary. Something like TWRP can be installed on many devices, which provides additional options including backing up and restoring your phone's ROM (OS, kernel, data, etc.).
You should be able to Google "<device> recovery mode" to find instructions for your particular device.