What you mean by root ? warranty , advantage, disadvantage , leagal rooting. And which device are rooting easly

The term "root" comes from the Linux world. The root operation in Android is much the same as it is there: it gets you full access to the entire file system, making you a "superuser". This allows you to make profound adjustments and changes to the software.

But for novice users, it also poses a risk, because very important system files can be just as easily deleted as less important files. This is one of the main reasons why Android smartphones are not rooted by default: to avoid innocent but catastrophic mistakes.

Root access is often gained by using a script that you install in Android's download mode, which automatically executes itself, granting root access in the process. This script often uses a vulnerability in the operating system kernel by performing a so-called "privilege escalation.

As we said above, root privileges turn you from a regular user into a superuser with access to the full Android system. The simplest way to extend superuser permissions to apps is with a tool called SuperSU.
You can do this on a case-by-case basis, for specific situations, or on a more permanent basis, to allow an app to help you take complete control over your Android. You can withdraw superuser permissions at any time, too.

Warranty-

Generally speaking, rooting your Android will void your warranty, but it's not always so clear cut. Some devices, such as those in the Nexus line from Google, are pretty much designed to be rooted and modified by Android developers in order to advance the platform. You can't expect the same leniency from all OEMs.

rooting Android legal-

That depends entirely on where you live. In the US, the modification of digital things like the software of mobile devices is covered by the Digital Millennium Copyright Act (DMCA). Critically,
some exemptions to that law exist courtesy of the US Copyright Office and they include things like rooting.
So, while rooting in the US is technically illegal under the DMCA, one such exemption to the DMCA makes rooting Android devices legal “at least through 2015”. While this was a big win for fans of digital freedom back in 2012, the legalities of the act are still to be completely ironed out even now, in 2016.
For those in Europe, the case is much clearer: the Free Software Foundation Europe (FSFE) has worked intensively around the root issue and, after a detailed examination, came to the conclusion that rooting a device and installing unofficial software does not impact your rights to a hardware warranty.

The good news for those in Europe is that the burden of proof lies on the retailer or manufacturer to prove that it was a custom ROM or the root process itself that caused the problem.
In the US, things are much less well defined and basically come down to a mishmash of manufacturer, carrier and retailer positions. The simple answer is that you shouldn't root unless you're happy to void your warranty.

How do I root my smartphone?

Because the process used to root individual devices varies so greatly and changes so quickly, there's little point in providing dedicated links to tutorials on AndroidPIT. The simplest thing to do is hit the search button and look for a current guide. But read on for some general rules.

How to root Samsung devices

Because Samsung devices are supported by the excellent Odin software tool, rooting Samsung devices is generally very simple. All you need to do is download the Odin program on your computer and flash the latest Auto-Root tool from the developer Chainfire.
Other root methods for Galaxy devices frequently come up and occasionally CF-Auto-Root takes a while to appear for a new Galaxy device, but it's generally one of the better and more reliable methods if you're at all unsure about the process.
Find out how to root the Samsung Galaxy S7 and S7 Edge at the link.
It should be reiterated here that Samsung includes a flash counter on its devices. This means that if you root your Galaxy device and flash anything on it and then have to return it for repair, Samsung might refuse your claim due to modifications being made that are not covered under warranty.

How to root LG devices

LG devices don't need a third-party program like Odin to root. All you need is a computer and some ADB commands with which to run a script or you can use a tool called One Click Root, if you're a little scared of the whole ADB thing.
LG notably provided official instructions for unlocking the LG G4 bootloader back in the middle of 2015 but with the caveat that you'll be voiding your warranty if you do so.

How to root Sony devices

Depending on which kind of Sony phone you have – one with an unlockable bootloader or not – the root process will be slightly different for you.
 If your bootloader is officially unlockable it's much easier to get a custom recovery on your Xperia and then make further modifications by flashing zip and image files.
You simply need to check Sony's list of Xperia devices with an unlockable bootloader and, if your device is on the list, find a custom recovery for it.
 We'd recommend TWRP, which has a convenient supported devices page to make the search that much easier.
 From here, all you need to do is grab a SuperSU exploit and flash it through recovery.