How to root Android?

Android rooting is the process of allowing
users of smartphones, tablets, and other
devices running the Android mobile operating
system to attain privileged control (known as
"root access ") within Android's subsystem.
Rooting is often performed with the goal of
overcoming limitations that carriers and
hardware manufacturers put on some
devices, resulting in the ability to alter or
replace system applications and settings, run
specialized apps that require administrator-
level permissions, or perform other
operations that are otherwise inaccessible to
a normal Android user. On Android, rooting
can also facilitate the complete removal and
replacement of the device's operating
system, usually with a more recent release of
its current operating system.
As Android derives from the Linux kernel ,
rooting an Android device is similar to
accessing administrative permissions on Linux
or any other Unix-like operating system such
as FreeBSD or OS X.
Root access is sometimes compared to
jailbreaking devices running the Apple iOS
operating system. However, these are
different concepts. In the tightly-controlled
iOS world, technical restrictions prevent
1. installing or booting into a modified or
entirely new operating system (a "locked
bootloader" prevents this),
2. sideloading unsigned applications onto the
device, and
3. user-installed apps from having root
privileges (or from running outside a secure
sandboxed environment).
Bypassing all these restrictions together
constitute the expansive term "jailbreaking"
of Apple devices. That is, jailbreaking entails
overcoming several types of iOS security
features simultaneously. By contrast, only a
minority of Android devices lock their
bootloaders—and many vendors such as HTC ,
Sony, Asus and Google explicitly provide the
ability to unlock devices, and even replace
the operating system entirely. [2][3][4]
Similarly, the ability to sideload apps is
typically permissible on Android devices
without root permissions. Thus, primarily the
third aspect of iOS jailbreaking, relating to
superuser privileges, correlates to Android
rooting.