Excel Spreadsheet File
Microsoft Excel uses the format XLS for its spreadsheets up until Excel 2007 which then used a XML based format that is still in use today in the latest versions.
Excel files are capable of storing graphs, charts, formulas and images, data is stored in rows and columns which have these own special identification.
An example of this is A5 which means the first column and the fifth row.
It is also possible to embed macros in excel files to perform certain functions but it is possible that this can be used for putting malicious code in to your computer so unless the source of such a file is trustworthy scan the file before opening.