For technical purposes a zone is a logical path through which users gain access to a web application- the URL is a common public face of a zone. The purpose of the zone is to let you into a web application, or other SharePoint application. Most people don’t ever think about the zone- they envision a URL directly accessing the web application, but this idea completely skips over the idea of a zone. However, a web application can have multiple URLs which all lead to the same place and use the same zone.
With the technical details the concept becomes complicated, and it’s very hard to make sense of everything. To truly understand zones, applications, and URLs let us take a real world example: a building. At first it seems as though a building has absolutely no correlation between these three unrelated topics, however I will show you it is one of the best analogies!
First you have the web application, which for us is the building you are attempting to enter:
The entryway into the building is the URL, and as we all know a building has multiple entrances which lead to the same building:
Once you enter into the building (web application) you are identified (authenticated), and there may be certain restrictions (policies) put in place.
A web application can have multiple URLs which lead to the same web application. Once a user enters the web application they are authenticated, and policies may be in place. In terms of our examples this means a building has multiple entrances, once you get to the door you are identified, and the building may have policies on what you can and cannot do once you enter.
Although zones and URLs are a complex task to understand, if you take a step back and think about the analogy to a building it becomes a crystal clear topic!