As we look deeper into the differences between XLS and XLSX, we can see that the way the information is stored is vastly different for both XLS and XLSX formats. XLS is based on BIFF (Binary Interchange File Format) and as such, the information is directly stored to a binary format. On the other hand, XLSX is based on the Office Open XML format, a file format that was derived from XML. The information in an XLSX file is stored in a text file that uses XML to define all its parameters.