A file extension, sometimes called a file suffix or a filename extension, is the character or group of characters after the period that forms a complete file name. Some common file extensions include PNG, MP4, PDF, MP3, DOC, SVG, INI, DAT, EXE, and LOG.

The file extension helps an operating system, such as Windows or macOS, determine which program on your computer the file is associated with.

For example, the file notes.docx ends with docx, a file extension associated with Microsoft Word on your computer. When you try to open this file, Windows sees that the file ends with a DOCX extension, which it already knows Word should open.

File extensions often also indicate the file type or file format of the file, but not always. The extension of any file can be renamed, but that will not convert the file to a different format or change anything about the file other than this part of the name.