Format XLS described as Excel Binary File Format was developed by Microsoft. He served as default format for Microsoft Office Excel until 2007. This format was precursor of XLSX and XLSB format which are also formats of Microsoft Office Excel. One of the advantage of this format is that he supports tables, functions, macros and etc. Main disadvantage is that he´s not open to public, so he can´t be freely editable and he´s no more developed.
Since 2007 Microsoft company created new file format for Excel, which was later on used as primary file type, which is Office Open XML. He´s fully replacing the old one and is much more better, because of its foundation on XML.
Nowadays there are many programs, which can open files saved in this format. First of all there is Microsoft Office Excel, which even after change of default format can still open files with XLS extension. Next program is Apache OpenOffice.org Calc. He´s free alternative of Office Excel. Third best known program is LibreOffice Calc which is adaptation of Apacha OpenOffice.org Calc. Last application which is for free is Excel Online. That was also created by Microsoft, but you need Microsoft account connected to OneDrive to run and use opening and editing of .xls files in Excel Online.