When it comes to something new like rooting, it is understandable people have many questions to ask. This post tries to answer some of the more common ones for your benefit:
a) How is rooting, flashing ROM and unlocking different from each other?
Very often though people may understand the broad meanings of terms, they want to know specifics as some practices like the ones mentioned above may seem similar and being performed simultaneously.
The unlocking process is usually the first step when you unlock the bootloader that enables you to do a custom recovery. Here on, you can either get root access or use a flash ROM. Though it is not necessary to enable root access in order to flash ROMs, most custom ROMs do come with access that is already built-in.
The term unlocking refers only to the bootloader and not to the phone as that is a different activity altogether.
b) Is the process of rooting my phone illegal?
The process certainly is not illegal and the best part is that you can unroot your device when you wish to as well and get back to stock default settings.
c) What about the warranty on my device?
Rooting will most certainly void your warranty. In fact even when you unlock the bootloader, you will void your phone warranty. But as mentioned above, you may unroot your device if you do not find value in rooting and get back stock settings to roll back the warranty as if nothing has happened. You need to read and follow steps carefully to do the rooting and unrooting properly.
d) Will rooting brick my device?
There is always a risk factor linked to these processes and you would have to take it. The possibility does exist if you do not follow instructions. As compared to flashing a custom kernel or radio, rooting is relatively safer as long as you stick to guidelines. Bricking is the ultimate price you would have to pay and is different from any other booting or loop issues that can be resolved.
e) Are there phones that are unrootable?
That is not likely as most of the phones can be rooted with some software or the other. Even those that have received an update are susceptible to rooting within a few months of their release.
f) Will I continue to receive OTA updates?
Unless you have rooted your device with a custom ROM, you will continue to receive these OTA updates. But do not download such updates as that would take your phone back to the earlier stage and you might not be able to re-root it. There are applications like the OTA Rootkeeper though that will enable you to maintain your rooted status. But flashing custom ROMs will not help you get these OTA updates in any case, but you may continue to get updates specific to the ROM installed.
Hope the above questions with answers have served to satisfy your own curiosity with regard to rooting.