File PPTX was created by Microsoft company for their Microsoft Office PowerPoint program. Its extension is .pptx, which means PowerPoint Open XML. Basically it is Open XML format saved into zip file. He´s almost identical with .ppt, that´s why he can handle with pictures, texts, links, videos, music, etc. This format was developed especially as replacement of default one which was using binary format and he tends to be open file type. So he has to be freely editable.
Its first use was in 2007 in office suite Microsoft Office 2007, specifically in Microsoft Office PowerPoint for Windows, a year later was released for Mac OS in Microsoft Office PowerPoint 2008. Since that he was used as default format for this program. Default application for opening these files saved in this format is Microsoft Office PowerPoint developed by Microsoft.
But there are many more alternatives which are for free. For example there is OpenOffice.org Impress, which can open this file format, like at the .ppt file type. Microsoft has also one free alternative – that´s Microsoft Office PowerPoint Online, which is web application. You need Microsoft account to run it.