SharePoint Web Applications, Collections, etc. Defined

More for my own reference and to remember:

Adding a SharePoint Web Application
Creating a new Web application is the most common option. When creating a new SharePoint Web application, you create a new database to store data, and define the authentication method used to connect the SharePoint application to that database.

Extending a SharePoint Web Application
Choose Extend an existing web application if you need to have separate IIS Web Sites that expose the same content to users. This is typically used for extranet deployments where different users access content using different domains. This option will reuse the content database from an existing Web application.

Site Collections
Only one at the root, the rest will be under /sites (by default, you can create your own paths). Site collections have their own quotas, owners, usage reports, etc.

Tips on Sizing –

WSS 3.0 Backup Time Metrics
15GB stsadm backup < 1 hour
15GB stsadm restore = 1 hour
15GB stsadm delete site collection = 19 minutes