This explains Excel Services pretty well:
http://office.microsoft.com/en-us/sharepointserver/HA101054761033.aspx
Excel Services is part of MOSS 2007.
In short, there are three parts of Excel Services:
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Excel Calculation Services – this is under the hood, does the math and formulas and stuff
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Excel Web Access – this allows you to see a workbook in a web part that looks and acts similar to excel
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Excel Web Services – this is for developers to make custom stuff
Tags: Excel Services, excel web access, MOSS 2007, SharePoint