If you follow Chris Webb’s blog (and if you’re a BI practitioner in the Microsoft space – why aren’t you following his blog?) you may know that I have an interest (Chris calls it an obscure enthusiasm) in developments surrounding Excel and its various online incarnations. One aspect of this that I have always struggled with is the difference between “Excel Services” and the “Excel Web App”. Today, while helping Chris test some of the forthcoming SharePoint/Excel 2013 functionality, I stumbled across a very useful help page that goes some way to explaining it: Comparing Excel Services in SharePoint to Excel Web App.
I don’t expect that resource to be around forever (because it begins http://officepreview.microsoft.com) so to save you from clicking on a dead link here is a screenshot of the entirety of that help page:

I’m putting this up here for my own edification as much as anyone else’s but hopefully it proves useful to some of you too.
@Jamiet