Android has changed the future of mobile phones; there would be no doubt if we give credit of vast popularity of smartphones to Android OS.
Android has changed the experience of Tablet & Smartphone. Before the Android Apple was the only giant in the field of Smartphone by Iphone and Ipad. After Apple Blcackberry also come in the market but it was considered for only business and corporate world. But Android become famous in every generation young, old, gamers, children, and business class it has choices for everyone.
Android has very fresh and customisable User Interface; you can turn it new every day by changing the free Launchers and many thousands of themes.
The most important thing that make android most popular is its vast apps store Google play (previously market).
There are more than 3 lacs of apps and continually increasing. You can do whatever you want through these apps the most popular are free ones.
Other thing that make it most popular is that its open source in nature. Android is based on Linux Kernel that is an open source frame work already popular in the world.
In your android phone you can install many versions of open source e.g. Cyanogenmod or MIUI, these are called custom ROMs.
The android OS that pre-installed with your mobile is called stock ROM in words of developers and the open source android OS just like Cyanogenmod are called custom ROMs. These OS are made by volunteer developer’s community to us. There are many flexibility, facilities and tweaks in custom ROMs. You can overclock , change and replace system files, can see hindi fonts, disable and enable features those are not available in stock ROMs.
Before installing these custom ROMs your mobile should be rooted.
Rooting is just like gaining administrator access in your windows OS for computers, you can install many system files and can change them as per your requirement that is called rooting. Actually your preinstalled OS come with blocked OS to ensure security of your mobile from viruses and other vulnerability that can change your system files.
Besides these Android have flexibility that you can update its firmware (e.g. from Android 2.1 to 2.2, 2.3 to 4.0 etc.) without losing your settings and apps.
Android phones have started very low range of smartphones e.g. Samsung Galaxy Y that is starting approax 7000/- only and many Indian mobile companies has launched even low budget phone below 7000/- also like spice, karbon, Lava, intex etc.
If we give most of share of credit to popularity of Android than it must be Samsung Galaxy Phone and Tablet series that become the most selling mobile brand in the world.