This is an excerpt from this link --> http://microsoft.com/windowsxp/whichxp.asp
Windows XP Professional is best for people who connect to large networks, such as a school or office network, since it allows you to join and be managed by a Windows domain. More about joining networks.
AFAIK, there is no way to log on to a domain from Home Edition, there may be a hack out there somewhere to do it though.