Excel Services is a SharePoint technology that extends Excel by using server technology. It lets a user access a workbook from the browser, while the server manages security and storage for the workbook and does any calculations that might be required. Users can navigate or sort and filter data in the workbook in the browser.
The workbook author can designate certain cells in the source workbook as parameters, which enables users to input values for run-time calculations. If the user’s computer has Excel installed and the user has appropriate permissions, the user can capture and save a snapshot of the workbook in the browser, or download the workbook and work on it in Excel.
Excel Services has three main components that work together to publish a workbook on a SharePoint site:
1. Excel Calculation Services is the "engine" of Microsoft SharePoint Server 2010. It loads the workbook, calculates in full fidelity with Microsoft Excel 2010, refreshes external data, and maintains sessions. Excel Calculation Services performs calculations on the server; users cannot access functions and formulas directly.
3. Excel Web Services provides an application-programming interface (API) that developers can use to build custom applications based on the Excel workbook.
Because Excel Services is a SharePoint technology, you can take advantage of features such as security and access management, server-based performance management, and scalability.
Source : https://support.office.com/en-us/article/getting-started-with-excel-services-and-excel-web-access-fed3586d-b150-4819-a67e-14529c974387