What is the difference between the standard and advanced version?
The Zimbra Standard Version is a traditional web-based email system.  It functions as a normal web page, with pages needing to be loaded separately as clicked.  It will work on virtually any web browser, with moderate speed.  While it does not have many of the bells and whistles that some consumers have come to expect, it is basic and functional, and will provide access to a customer's messages.  
The Zimbra Advanced Version is a more modern system, designed with fast computers in mind.  It uses AJAX (a relatively recent web-based technology) to deliver more dynamic pages and allow for things previously impossible in web systems.  The Advanced Version builds on the Standard Version with such features as right-click context menus, spell check, and preloading.  While the Standard Version can highlight spelling errors, the Advanced Version will actually provide suggestions for replacement if you right click the misspelled words.  The Advanced Version tends to look more seamless in action, because the page does not blank out when something is clicked on.  Content is simply loaded in place with very little visual change.