Up until the 2007 version, Microsoft Excel used a proprietary binary file format called Excel Binary File Format (.XLS) as its primary format. Microsoft Excel 2007, along with the other products in the Microsoft Office 2007 suite, introduced new file formats. The first of these (.xlsx) is defined in the Office Open XML (OOXML) specification.
Here are some Excel conversion tools, Convert data between Excel and other file types, between Excel and databases, Merge Excel files, Split Excel files.