Bob Mixon has posted a great article about an important choice when architecting your SharePoint site hierarchy: should you use one site collection, or multiple site collections. 

The article describes the pros and the cons of having one or multiple site collections. Some important concepts:

  • a site collection is a unit of information belonging together; if information should be isolated/secured, it could be a separate site collection;
  • having multiple site collections might require some additional development (e.g. packaging content types or site columns that you want to reuse in different site collections in features);
  • a lot of built-in web parts only work inside a site collection, not across site collections;
  • it’s good to have separate site collections for a corporate, governed intranet and the wild wild west of collaboration  :-)

Excellent reading for everyone managing a SharePoint site.