I'm new to SharePoint and I have a question. If you already have an existing intranet or extranet infrastructure and you want to create(/expose) another one, should I create a new site collection, a new web application or extend the existing web application to a new zone?
It can depend on circumstance. But normally its best to create a new site collection. Web applications are quite resource hungry, the more you have the more load you are placing on the farm.
Information architecture with SharePoint is not easy when you're just starting out with the product. The used terminology alone can be very misleading.
A very good resource for knowing the differences between sites, site collections and web applications can be found here:
In general you would want to create more site collections within the same web application for scalability reasons. A site collection is a boundary for a lot of things like branding, navigation and security. There are of course many reasons to create additional web applications too.