LiveUpdate obtains program updates and definition updates for your computer by using your Internet connection.
Program updates are minor improvements to your installed product. These differ from product upgrades, which are newer versions of the entire product. Program updates are usually created to extend the operating system or hardware compatibility, adjust a performance issue, or fix program errors. Program updates are released on an as-needed basis.

LiveUpdate automates the process of downloading and installing program updates. It locates and obtains files from an Internet site, installs them, and then deletes the leftover files from your computer.
Definition updates are the files that keep your Symantec products up to date with the latest antithreat technology. The definition updates that you receive depend on which product you use.

The type of definition updates that each of the Symantec products receive are as follows:

Norton AntiVirus, Norton AntiVirus Online	Users of this product receive virus and security risk definition updates, which provide access to the latest virus signatures and other technology from Symantec.
Norton Confidential	Norton Confidential users receive phishing definition updates, which are used to help identify these risks while you browse the Internet.
Norton Internet Security, Norton Internet Security Online, Norton Security Online	In addition to the virus and security risk updates, users of this product receive definition updates for security protection. For the products that contain protection against phishing, users receive definition updates against phishing. The security definition updates provide the latest predefined firewall rules, the updated lists of applications that access the Internet, and Intrusion Prevention signatures. These lists are used to identify unauthorized access attempts to your computer.
Norton 360, Norton 360 Online	Users of these products receive virus and security risk definition updates, which provide access to the latest virus signatures and other technology from Symantec. In addition, users of these products receive definition updates against phishing.

If you are interested about this subject and virus definitions, also I recommend you to read about the Types of virus definitions available for Norton Internet Security.