Dave,
So WCS (Wireless Control System) is a software based global monitoring and management solution, you would use WCS to view alerts, alarms, logs, as well as configure controllers and access points. WCS enables you to create maps of your facility and place the AP's on those maps and it will generate predicitive heatmaps and other funtions as well as displaying client locations provided you have a Location Appliance or a Mobility Services engine with Location Services.
A WLC (Wireless Lan Controller) is a network device that is used to control and manage access points, in the lightweight LWAPP/CAPWAP wireless model you must have LWAPP access points and a WLC in order to run the access points. The WLC allows you to configure and manage access points that are attached to it up to it's AP limit (license based), as well as view the trap logs. It does not however allow you to see location information, heat maps, and many other bits of information available in WCS.
This is a brief overview of what the difference is between them, there are a great deal more features and functions with WCS that do not exist in the WLC, but WCS must have WLC's and AP's to be useful where a WLC doesn't need a WCS for basic funtion.
Hope this helps... Please rate useful posts.
Thanks,
Kayle