An S-Control is a lightweight web application/tool that is hosted by Salesforce.com, but executed on the browser… much like Javascript. An s-control can contain any type of content that can be displayed or run in a browser, such as a Java Applet, an ActiveX Control, or HTML/JavaScript/AJAX. S-Controls are activated via Custom Links, Web Tabs, Web Apps or HYPERLINK custom formula fields, Detail Pages of records, and Custom Buttons in salesforce.com.
Here are some Salesforce.com resources to better familiarize you with S-Controls:
S-Controls for Dummies
http://blogs.salesforce.com/dreamforce06/2006/09/scontrols_for_d.html
S-Controls forum:
http://community.salesforce.com/sforce/board?board.id=ajax_toolkit
Demystifying S-Controls and Ajax:
http://blogs.salesforce.com/dreamforce06/2006/09/demystifying_sc.html
The Future of Composite Apps: S-Controls and Beyond
http://blogs.salesforce.com/dreamforce06/2006/09/future_of_compo.html