Q) We are in a wss3 learnring mode. We want to setup a main page ("portal") of interest to several departments. At the top of that page would be a series of tabs, one for each department. Now, DepartmentA may need to access some items in DepartmentB's and DepartmentC's, etc. sites (subsite, web page?) but not all items. (Some of the items in DepartmentB, DepartmentC are sensitive to each department).There is no reason why someone in DepartmentA would know exactly where the information of interest to them resides in Department B's site. That means they would end up be "poking" around a lot in DepartmentB looking for it. So DepartmentB better be locked down really tight from a security perspective. But creating, testing, documenting, and upgrading such a security setup seems to quite involved. I needs some ideas how to implement such a setup or perhaps you have an entirely different idea how to accomplish the task.
A) Access permission for your site requires lots of planning, and You can create windows security groups and permission group in sharepoint and define permission levels for each department website and then add users to the group, this will ensure that your users access only their respective department site.
For more information check these linksPermission levels and permissions http://office.microsoft.com/en-us/help/HA101001491033.aspx
Managing security and permissions http://office.microsoft.com/en-us/sharepointserver/CH101248581033.aspx
Understand groups and permissions on a SharePoint site http://office.microsoft.com/en-us/sharepointserver/HA102371171033.aspx, This link will discuss about creating groupd and permission for your site.
About managing SharePoint groups and users http://office.microsoft.com/en-us/sharepointserver/HA100215791033.aspx, this link will discuss about Assigning users and GroupsRoadmap for giving users access to SharePoint Server 2007 sites and site content http://office.microsoft.com/en-us/sharepointserver/HA102486891033.aspx, this link discuss about how to Import users into Office SharePoint Server 2007, Configure permission inheritance,Add users to default SharePoint groups, Manage unique permissions for site content