Microsoft Office Excel Viewer is an application developed to open and display the Excel files. You can use it to view the content of XLS, XLSX, XLT and another less popular file formats. It doesn't support CSV files.
The program doesn't allow to edit, save or make other operations except for printing. So, it can be used just as a document viewer.
You can copy the content of the documents into another files, but you cannot edit it and save it. For editing you need to use the Microsoft Office suite which is paid or another similar, but free application such as OpenOffice and LibreOffice.
Excel Viewer is available as free download for Windows 32-bit and 64-bit.