A file with MPP extension is Microsoft Project data file that stores information related to project management in an integrated manner. It is proprietary file format developed by Microsoft as file format for Microsoft Project (MSP) which is a project management application software. Besides MPP, MSP supports other file formats as well like project XML schema. Several APIs and applications provide the facility to convert MPP file format to others.
Files with XLS extension represent Excel Binary File Format. Such files can be created by Microsoft Excel as well as other similar spreadsheet programs such as OpenOffice Calc or Apple Numbers. File saved by Excel are known as Workbook where each workbook can have one or more worksheets. Data is stored and displayed to users in table format in worksheet and can span numeric values, text data, formulas, external data connections, images and charts.
You can also convert MPP into many other file formats. Please see the complete list below.