It's a site, not an area
If you remember the 15 minutes it took to learn SharePoint Portal Server 2003, you'll remember that the 2003 version referred to different user pages as "areas." It apparently caused too many headaches on both sides of the IT office door, so under the new MOSS we get to refer to them as "sites." This makes more sense to the geeks, though users probably won't care one way or the other. To them, sites tend to show up as tabs in the MOSS user interface, just like what they're used to in applications such as Office 2007. In fact, most of the front-facing SharePoint 2007 UI has been specifically and in some cases painstakingly designed to look as much like Office 2007 and general Windows applications as possible. The downside here is that if you get really customized with SharePoint user sites, you're pretty much forced to use SharePoint Designer since it'll have the UI templates necessary to maintain this look and feel.
In case you missed it, SharePoint Designer is half of what Microsoft FrontPage morphed into when it died and was reborn with the Expressions label. Designer is a similar-looking Web design tool, but built specifically for SharePoint users, and although it's probably possible to build SharePoint pages using standard Web design tools, these would only have basic functionality. Only Designer will allow easy access to core SharePoint features, especially the communication, collaboration, and workflow tools. And "easy access" is critical to SharePoint's overall sales pitch; developers have plenty of meat on the SharePoint bone, but most of the standard features, including site customization, need to be modifiable by non-IT users. So administrators looking to commit to SharePoint will need to factor in Designer license costs as well. The upside is that this really does make it easier on end-users, especially if they're been pretrained on Office.
Configuring the initial top-level site isn't hard, but does mean you'll need to address the users and permissions question again. On initial examination, ITmeisters might complain that SharePoint doesn't use predefined users and groups from an existing AD (Active Directory) tree. But there's a reason this software should have its own usage structure: the fact that it allows users to build and assign work sites of their own, complete with users and permissions they can assign themselves. Going back to AD would mean that IT would have to get involved with every new SharePoint site creation.
Customizing a top-level site isn't difficult, but you can't forget to hit the Site Actions button and track down to People and Groups. This is where you can add the home users for the top-level site as well as the member users of this MOSS server. That last one is important for their initial log-on experience. It's not as nightmarish as it first sounds, since Microsoft thoughtfully included an "Add All Authenticated Users" option, which will automatically import every user currently part of the SharePoint server's domain.
Users can now create work sites of their own beneath this top-level site. Each site gets its own template and can assign its own users complete with permission specific to that site. The site creation tools have easy cherry-picker options to find the users you're looking for, turn them into groups, and assign permissions on either basis.
End-users get a top-level site, too, called MySite. It is automatically generated when a user accesses the default portal site, is authenticated, and then hits the MySite button. MySite has a number of tools, but its purpose is to summarize all the MOSS team sites to which the user belongs, including new information alerts and task roll-ups. Users can also access MySite either via the SharePoint server or as part of their Outlook Today views.