Jailbreak is a term use in iOS. In android, we root our phones. Why? Few reasons, to install custom ROM, it is mainly because that phone no longer receiving update, so have to rely on xda. 2nd, to over clocked your phone. 3rd , dislike skinned ui, example HTC sense , Samsung touch wiz and more. So they would go for pure android aka vanilla. Or some prefer cyanogen version. Anyway, welcome to android, you can find almost all information about your android phone here. Have a nice day
Sent from my Galaxy Nexus using xda premium