I posted about this in the past, but I can’t find the post (how embarrasing). No need… Heather explains it very well:
When designing a portal, keep the major areas in mind in relation to your URLs. SharePoint allows 20 areas (one being the Home) to be formatted nicely in the URL. For example, http://ThePortal.com/HR or http://ThePortal.com/Training . Once you hit the end of your 20 allotted areas, the URL will start organizing the areas into buckets.