Bootloaders basically holds the instructions to bootoperating system kernel and it runs before any piece of software on a androiddevice. As a result it is extremely processor specific and every motherboardhas its own bootloader. Bootloaders usually contain several ways to boot the OSkernel (safe mode, normal mode etc.) and also contain commands for debuggingand/or modifying the kernel environment.

Every Android phone has a bootloader that instructs the operating system kernel to boot normally. But as Android OS is an open source OS and is available on a variety of different hardware, every manufacturer has their own version of bootloader specific for the hardware.

When the device is turned on, basically the bootloader takes the instruction to turn on the phone by passing off control to the boot image (the part of the disk that holds the start-up files of the phone).  The boot image loads the phone's kernel, and then loads Android, following the instructions found in those files.  

If you have a locked bootloader, you can only flash boot images that have been digitally signed by the manufacturer.  You can't build a custom one and flash it tothe phone.  The recovery partition is the same way is checked for the right signature.

As the user can load only the software that has been approved by both the manufacturer and the carrier, it is easier for them to keep the phone secure and working as intended. This is the perfect solution, except the fact that by the time a security or bug fix is released, new issues have been found.  It is a never-ending cycle, and locking the phonedown to approved software helps the carriers and manufacturers administersupport it with minimum effort.  Butanother important fact is, not everyone who owns an Android phone is has ideahow to load patches and fixes by them self.  The manufacturer who made the phone knows best when it comes to support.And yes they provide the warranty for all the phones they made.

A phone with an unlocked bootloader, one can Root, cookcustom ROMs, port of apps of other device -- all the things many of us loveabout Android.  And also, unlocked bootloaders mean custom kernels, over clocking, USB host, and all manner of other goodies that's pretty much difficult to manage on phones with lockedbootloaders, as well as an easy way to load it on your own phone.

One has to be extremely careful with unlocked bootloader as there are potential threats involved with it. First of all your warranty willbe void on most of the cases. You may stop receiving OTA updates. Installing an unoptimized ROM may cause performance issue and sometimes may break your phone.One may end up installing malware application which will take control of yourphone without your knowledge and steal your personal data like bank accountdetails and passwords.

Very few people out of the 400,000 Android devices activatedevery day are users who have any idea what a bootloader is. Their main purposeis to own a “Stunning” looking phone at good “Price”. It’s their showoff andthey can do many smart things apart from phone call and SMS. They cango social with Facebook, WhatsApp, Skype and many. They will get addicted to music with thousands of songs stored in the phone. They can go creative with a decent phone camera. Office executives can do some of their office work with a smart phone – like mail, document etc. A common user may add as many featuresas made available by the manufacturer and Google Play Store.

The people who do care about unlocking bootloader arebasically passionate people who want to have complete control over their phone to decide what software goes on their Android phones. They are the coders, themers, developers and hackers who endlesslytinker and improve the system they were given, and turn it into something better and ready to take risk. Who don’t care about warranty and services and have the confidence that if something goes wrong they can take care of it by their own.

Share your understanding and knowledge on the subject.