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?
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.