An open format is a file format with an openly-published specification that anyone can use. It is the opposite of a proprietary file format, which is only used by a specific software company or application.
Open formats make it possible for multiple programs to open the same file. For example, if you create a plain text (.TXT) file in a text editor like Notepad and open it with another application like Microsoft Word. Since plain text files store data in an open format, they can be opened by any text editor or word processor.
Conversely, a file saved in a proprietary format may only be opened by the corresponding application. For example, only Apple Pages can open a .PAGES document. Only Adobe Premiere can open a .PRPROJ file.
In recent years, many software companies have moved from proprietary formats to open ones. Microsoft switched from proprietary Office file formats (.DOC, .XLS, .PPT) to open formats (.DOCX, .XLSX, .PPTX) in 2007. The new "Open XML" Office formats use standard Zip compression and contain data stored in plain XML. Any application that supports the publicly-available Open XML specification can open them. In 2008, Adobe made PDF an open standard. The International Organization for Standardization (ISO) now maintains the .PDF format specification.
Partially Open Formats
Some file formats are "partially open," meaning the specification is made partially available for public use. Other applications can open these files, but they may have limited editing and saving capabilities. Examples of partially open formats include Photoshop documents (.PSD), CorelDRAW drawings (.CDR), and AutoCAD drawings (.DWG).