Welcome to Android Central. Your question covers a lot of territory. Android OS is based on Unix/Linux and has a different structure then most people are used to. Root means to have superuser, or administrative access and control over the device. Normally you have very little actual access to the inner workings of the phone. Obviously them, unrooted means the opposite, or how the device comes to you. Pros are much greater control over the operating system, cons are much greater control over the operating system. A superuser who knows what they're doing can improve the operation of the device. A person who doesn't know what they're doing may turn the device into a brick. Which is better is up to you