Welcome to the forum!
Excel Services is a new product in the Microsoft Office SharePoint Server family. In a nutshell, it allows one to run Excel models (Workbooks) on the server. It comes with EWA (Excel Web Access) which gives very high fidelity access to the workbook contents over plain HTML/DHTML.
On top of that, the server also supports an API and the ability to expand on the function library by using User Defined Functions written in . NET.
MSDN resources: http://msdn2.microsoft.com/en-us/library/ms517343.aspx
The Excel Blog: http://blogs.msdn.com/excel/archive/tags/Excel+Server/default.aspx
LuisBE on Services: http://blogs.msdn.com/luisbeonservices/default.aspx
Cum Grano Salis: http://blogs.msdn.com/cumgranosalis
Feel free to post comments with more resources!
Enjoy!