XLSX files as well as their predecessor, XLS, are produced by Microsoft Excel, which is part of the MS Office suite. The Excel format is a very useful tool that can create spreadsheets with data and complex mathematical formulas, among others.
The updated version replaced the first one in 2007 and contains a number of improvements. Business users turned to an open file format in terms of transferring input between apps, therefore Microsoft had no other option but to introduce the new extension. The letter ‘X’ stands for XML - eXtensible Markup Language that makes it easier for a computer to read and store data. Another advantage is that files are automatically compressed which significantly reduces their size.
Besides, file security while transferring it via the Internet is no longer an issue because XML data is stored as plain text. Users can open the new format in any other spreadsheet application, e.g. Open Office, Google Docs, or Apple Numbers for Mac users.
Those who prefer reading and editing files online might as well upload them via Google Sheets. Problems with opening the updated extension might arise only if you use an old version of MS Office (earlier than 2007).