Q. What is a jailbreak and rooting?
A. All bought iPhones, iPod touches, iPads, Android, Honeycomb, and Windows Phone 7 are delivered "in a jail", meaning that you cannot install any software on them that is not approved by the manufacturer. Jailbreaking (term used for Apple devices) means to modify your device so that it allows to run any software and customize your device. This means it is then your choice of what to install and how your phone should operate and look. The term "rooting" is the same concept, but used in relation to Android devices.
Q. Will Jailbreak or rooting my device void my warranty?
A. Yes manufacturers have the right not to honor their warranty if a device is jailbroken or rooted.
Q. Is the process reversible?
A. Yes the process is completely safe and reversible. If necessary your phone can be put back to its original state. You will lose any features and apps acquired through jailbreaking/rooting.